Armson Semi Teardown.
https://www.vintagerex.com/cgi-bin/index.cgi?action=viewmarker&marker=Armson%20Semi&m an=Armson
I know there aren’t a lot of these out there, but I thought this could help with the ones that do exist.
Most of this information comes from the work I have done on these lately or directly from Forest and all the help he has given me through many many emails.
Please feel free to correct me on any mistake or if you have something useful to add I’ll update the post.
Interesting old hand drawn parts diagrams.
Tools required:
4mm Allen wrench
5mm Allen wrench
Flathead screwdriver for stock screw for trigger shoe (or whatever is needed to remove trigger shoe).
10mm open ended wrench for powertube and expansion chamber.
Small punches and a hammer
O-ring pick
O-Rings:
Original sizes are all metric with exception of the ASA plug(tank o-ring). Some can be substituted with standard o-rings which is reflected below. I don't have a list of actual sizes of the o-rings so below is a best guess using calipers and trial and error for what has seemed to work best so far.
Barrel collet 70d 213
Rear cap o-ring 70d 210
Powerfeed plug 70d 3mm x 13.5mm ID
Powertube o-ring 70d 1.5mm x 13mm ID (70d 014 seems to work well here too)
Air transfer assembly o-ring 90d 2mm x 13mm ID (70d will work fine)
Velocity adjustment plug 90d 2mm x 13mm ID (70d will work fine)
ASA screw usually has nylon washer on it, but I have used a 90d 010 in place of a washer without issue.
In grip expansion chamber:
Top and bottom 1.6mm x 8.1mm ID
Internal seal o-ring for 2 halves 1.5mm x 15.5mm ID
CO2 baffles 1.5mm x 11mm ID
(Baffles can just be removed for use with HPA)
For a bolt bumper I use a 70d 211 o-ring. A smaller o-ring such as a 113 can be used and sit up under the bolt itself around the base of the powertube. I tried a softer 50d 211 o-ring and it was too soft and caused the bolt to not reset reliably with the stock bolt spring.
Back cap that covers the velocity adjustment plug, front barrel retaining collet, asa plug and barrel are shown removed.
2 barrel collet o-rings are pictured that hold the barrel in place once tightened down. I recommend making sure these have some oil on them. Also, when storing the gun you should loosen the collet. Somehow it seems to get tighter over time as the o-rings settle into place and can be difficult to loosen by hand after sitting. This is why many of the guns have tool marks all over the barrel collet. One of those rubber jar openers is very useful for both tightening and loosening the collet without tools.
Gun is shown with 5 screws removed that basically hold the whole thing together. These screws are:
Four M5x12mm
One M5x10mm dogpoint
Even though not exactly the same, 10-32 screws seem to be close enough to use in place of the M5 screws.
It is easy to find M5x12 screws to replace. The dogpoint screw for the barrel not so much. I have just resorted to using one of 12mm screws and filing down the end of it to a “dog point” and using a small washer to effectively shorten it to 10mm usable length.
Pic shows the front screw holding on the power feed and sticking through the body to help align the barrel and prevent it from turning.
Detent disassembly.
Roll pin size is 2mm x 10mm
Also pictured is the insert for the front grip screw. It’s a proprietary item and I have searched for something similar but so far come up empty. Only ones I have I got from PTP. It is M8-1.0 externally threaded and M5 internally threaded.
This bumper is found pushed all the way forward in the body. I have used tubing found at the local hardware store as a good replacement.
Valve assembled. Note the 70D size 210 o-ring to help keep the back cap from unscrewing itself and the 70D size 211 bolt bumper o-ring between the base of the valve body and the bolt.
Valve disassembled.
Use 5mm Allen key to remove velocity adjuster and air transfer assembly.
10mm wrench to remove powertube.
The valve also has one insert installed on the top and one on bottom for assembly. These steel inserts are M8-1.00 threads on the outside and M5 threaded on the inside. All the inserts I have found are too long and I have had to cut them down to size. I have also rethreaded the valve body to accept the more common M8-1.25 threaded insert with M5 internal threads. The one on top can be seen in pic below.
The velocity adjuster may or may not be drilled out. If using HPA, a drilled out adjuster allows for more volume and helps to get a wider range of velocity adjustment.
Air transfer assembly.
I have seen 2 styles. One on the left is the “updated” version most commonly seen.
One on the right with ball valve was installed on rental Semis I received from South Africa.
When reinstalling the black nylon washer, the indentation should face down away from the valve. The spring sits inside that indentation and the brass piece slides over the spring and sits inside the plug. I have tested with similar springs without issue in this part, so if you lose yours, just find one that is close to the original.
The brass piece is in very short supply. Try to be careful with that piece.
Powertube and bolt disassembled.
Bolt and rubber bolt tip. The NOS bolts are pretty grungy, but they clean up just fine. The polished one has also been modified for use with a freak bored barrel by sanding down the tip.
Disassembled bolt. Your bolt likely will not just come apart like this, but if you were wondering what was up in there and how it was held together…
Powertube seal. This is a urethane u-seal. Size 5x10x5mm. That is 5mm tall, 10mm OD, 5mm ID. Pic of where I found replacements.
Grip frame disassembly is pretty straightforward, just start knocking out pins with a punch until everything falls apart. It appears that the way these probably came from the factory was with the pins put in from the left or driver side. So the knurled end of the pin should be to the left. So typically when using a punch to remove the pins, you will hit the pin from the right side. Some of the holes on the rentals were worn out and elongated and so the pins were flipped the other way.
A 5mm Allen key is used to remove asa and a screwdriver to remove the trigger shoe. Use a 10mm wrench to remove the in grip expansion chamber.
You will notice the asa screw has either a nylon washer or oring on it, either works to seal the asa although the nylon washer will last longer. The screw has part of the threads ground off to allow air into the expansion chamber. This screw is an M6x20mm. It can be replaced easily if lost, but you will have to cut some groves into it to allow air to transfer into the expansion chamber.
Remove the expansion chamber using the 10mm wrench. Sometimes the cap comes off and the rest remains in the grip. Don’t worry, the inside of the top of the chamber has a 5mm hex head and can be screwed out separately. Internal CO2 baffles shown. I just left the baffles out for use with HPA.
Grip frame with all pins removed.
There are 2 knurled pin sizes.
Three 4mm x 20mm. Also found some that worked in a standard size .125 x .500.
Two 3mm x 13 mm
Remove the trigger shoe to allow the trigger to slide out the top.
The stock screw holding the trigger shoe on is a flathead M3 x 6mm
Safety spring and ball are hidden below the pin holding in the sear. Safety bb size is 3mm.
There is a sear adjustment set screw that is accessed through a hole in the front of the grip. Besides the asa threads, these are the only non metric threads on the gun. I think this was a later addition to the gun.
The threads on this set screw are 5/16-24. Use a 5/32 allen key or just the 4mm as its close enough and this should be easy to remove.
Closer picture of sear adjustment screw and spring. Spring has a brass cap on it. I have not been able to find replacements for this. So you might have to get creative if you lose yours. Whatever you might use, just ensure it is no wider than the O.D. of the spring or it won’t allow the sear to travel up and down. One thing I have done is use a 1/4” ball bearing and JB weld it to the top of the spring, which does work. The other thing I have done is used a brass dome solid rivet and cut it to the size and shape I needed, which also works well.
Trigger assembly has a sear release that is spring loaded inside the trigger. If necessary, use a small punch to remove this roll pin.
Pin size is 2mm x 8mm.
I have used alternate springs here just for testing such as detent and safety springs. Seems that as long as the spring pushes the release out far enough that it rests on the pin it works fine. Too stiff of a spring and you will notice a clicking noise when dry firing. That is this little metal piece catching the front of the sear. Not a problem, just kind of annoying.
There are no spares for the little metal sear release. Unless more are in South Africa and get shipped back to PTP. I had some spares CNC’d. So if you need one let me know.
Rainmaker helped get me the CAD file for the replacement part.
Pic of where the air transfer assembly connects to the grip. On some guns the edge of this hole is pretty sharp and you may notice it clip your air transfer assembly oring. Sanding it down a little to take that edge off helps prevent it from clipping the oring when being installed.
You can see the air transfer hole that leads down to the top of the expansion chamber. I have expanded this hole on most of my guns using the next size up drill bit, just to increase airflow a little. Obviously don’t go too big or it won’t seal.
Freak boring
The Armson barrels can be freak bored without any issues. I have had several of them done.
The bolt on the other hand will not work with inserts below .684 in my experience. Therefore it may be necessary to sand down the bolt tip, otherwise the bolt will stick inside of the smaller inserts. I am sure everyone will have a different method for doing this, I don’t have any fancy machine tools so I just basically turned my hand drill into a makeshift lathe and used my Dremel with a sanding wheel to sand down the tip of the bolt. You will need the last 1/4” or so of the tip of the bolt to be sanded down. I would just stop periodically and check it with my .675 insert.
This is also a good opportunity to take some fine grit sandpaper and polish the rest of the bolt.
Semi foregrip mod
I have added foregrips to most of my Semis. I think they are a good addition to the gun and they aren’t a permanent mod, so you can always return to the stock configuration. The foregrip just replaces the front grip screw.
The foregrip I found that works the best and was reasonably priced and easily accessible was one built for an A5 sold by Allen Paintball Products. It also had some milling that kinda matched the gun already.
I bought a handful of the APP A5 grips and took them to the local machine shop to turn down the top to fit in front of the Semi trigger guard. It’s a tight spot that even Automag grips won’t fit into. I was thinking maybe a standard SP gadget grip might work, but there aren’t enough of those floating around. (As of the time of this post, the A5 grip shown on the APP website is not the one shown below, but the one below is the one they have in stock).
The very top should be turned to a diameter of about .675”. That probably needs to extend down about an 1”, but I just had them turn it down to a spot that made sense using the existing milling which ended up closer to 1.3”. Next I drilled and tapped the top to accept M8-1.25 external threaded M5 internally threaded inserts. I bought some 25mm and 30mm M5 set screws to use as my front grip screw. Some of the foregrips were drilled a little deeper than others.
I like to add a 7/8” rubber tip made for furniture to the foregrip to just protect it from getting scuffed up. I didn’t even bother with Loctite as the tension from the set screw in the insert pretty much locks everything in place.
https://www.vintagerex.com/cgi-bin/index.cgi?action=viewmarker&marker=Armson%20Semi&m an=Armson
I know there aren’t a lot of these out there, but I thought this could help with the ones that do exist.
Most of this information comes from the work I have done on these lately or directly from Forest and all the help he has given me through many many emails.
Please feel free to correct me on any mistake or if you have something useful to add I’ll update the post.
Interesting old hand drawn parts diagrams.
Tools required:
4mm Allen wrench
5mm Allen wrench
Flathead screwdriver for stock screw for trigger shoe (or whatever is needed to remove trigger shoe).
10mm open ended wrench for powertube and expansion chamber.
Small punches and a hammer
O-ring pick
O-Rings:
Original sizes are all metric with exception of the ASA plug(tank o-ring). Some can be substituted with standard o-rings which is reflected below. I don't have a list of actual sizes of the o-rings so below is a best guess using calipers and trial and error for what has seemed to work best so far.
Barrel collet 70d 213
Rear cap o-ring 70d 210
Powerfeed plug 70d 3mm x 13.5mm ID
Powertube o-ring 70d 1.5mm x 13mm ID (70d 014 seems to work well here too)
Air transfer assembly o-ring 90d 2mm x 13mm ID (70d will work fine)
Velocity adjustment plug 90d 2mm x 13mm ID (70d will work fine)
ASA screw usually has nylon washer on it, but I have used a 90d 010 in place of a washer without issue.
In grip expansion chamber:
Top and bottom 1.6mm x 8.1mm ID
Internal seal o-ring for 2 halves 1.5mm x 15.5mm ID
CO2 baffles 1.5mm x 11mm ID
(Baffles can just be removed for use with HPA)
For a bolt bumper I use a 70d 211 o-ring. A smaller o-ring such as a 113 can be used and sit up under the bolt itself around the base of the powertube. I tried a softer 50d 211 o-ring and it was too soft and caused the bolt to not reset reliably with the stock bolt spring.
Back cap that covers the velocity adjustment plug, front barrel retaining collet, asa plug and barrel are shown removed.
2 barrel collet o-rings are pictured that hold the barrel in place once tightened down. I recommend making sure these have some oil on them. Also, when storing the gun you should loosen the collet. Somehow it seems to get tighter over time as the o-rings settle into place and can be difficult to loosen by hand after sitting. This is why many of the guns have tool marks all over the barrel collet. One of those rubber jar openers is very useful for both tightening and loosening the collet without tools.
Gun is shown with 5 screws removed that basically hold the whole thing together. These screws are:
Four M5x12mm
One M5x10mm dogpoint
Even though not exactly the same, 10-32 screws seem to be close enough to use in place of the M5 screws.
It is easy to find M5x12 screws to replace. The dogpoint screw for the barrel not so much. I have just resorted to using one of 12mm screws and filing down the end of it to a “dog point” and using a small washer to effectively shorten it to 10mm usable length.
Pic shows the front screw holding on the power feed and sticking through the body to help align the barrel and prevent it from turning.
Detent disassembly.
Roll pin size is 2mm x 10mm
Also pictured is the insert for the front grip screw. It’s a proprietary item and I have searched for something similar but so far come up empty. Only ones I have I got from PTP. It is M8-1.0 externally threaded and M5 internally threaded.
This bumper is found pushed all the way forward in the body. I have used tubing found at the local hardware store as a good replacement.
Valve assembled. Note the 70D size 210 o-ring to help keep the back cap from unscrewing itself and the 70D size 211 bolt bumper o-ring between the base of the valve body and the bolt.
Valve disassembled.
Use 5mm Allen key to remove velocity adjuster and air transfer assembly.
10mm wrench to remove powertube.
The valve also has one insert installed on the top and one on bottom for assembly. These steel inserts are M8-1.00 threads on the outside and M5 threaded on the inside. All the inserts I have found are too long and I have had to cut them down to size. I have also rethreaded the valve body to accept the more common M8-1.25 threaded insert with M5 internal threads. The one on top can be seen in pic below.
The velocity adjuster may or may not be drilled out. If using HPA, a drilled out adjuster allows for more volume and helps to get a wider range of velocity adjustment.
Air transfer assembly.
I have seen 2 styles. One on the left is the “updated” version most commonly seen.
One on the right with ball valve was installed on rental Semis I received from South Africa.
When reinstalling the black nylon washer, the indentation should face down away from the valve. The spring sits inside that indentation and the brass piece slides over the spring and sits inside the plug. I have tested with similar springs without issue in this part, so if you lose yours, just find one that is close to the original.
The brass piece is in very short supply. Try to be careful with that piece.
Powertube and bolt disassembled.
Bolt and rubber bolt tip. The NOS bolts are pretty grungy, but they clean up just fine. The polished one has also been modified for use with a freak bored barrel by sanding down the tip.
Disassembled bolt. Your bolt likely will not just come apart like this, but if you were wondering what was up in there and how it was held together…
Powertube seal. This is a urethane u-seal. Size 5x10x5mm. That is 5mm tall, 10mm OD, 5mm ID. Pic of where I found replacements.
Grip frame disassembly is pretty straightforward, just start knocking out pins with a punch until everything falls apart. It appears that the way these probably came from the factory was with the pins put in from the left or driver side. So the knurled end of the pin should be to the left. So typically when using a punch to remove the pins, you will hit the pin from the right side. Some of the holes on the rentals were worn out and elongated and so the pins were flipped the other way.
A 5mm Allen key is used to remove asa and a screwdriver to remove the trigger shoe. Use a 10mm wrench to remove the in grip expansion chamber.
You will notice the asa screw has either a nylon washer or oring on it, either works to seal the asa although the nylon washer will last longer. The screw has part of the threads ground off to allow air into the expansion chamber. This screw is an M6x20mm. It can be replaced easily if lost, but you will have to cut some groves into it to allow air to transfer into the expansion chamber.
Remove the expansion chamber using the 10mm wrench. Sometimes the cap comes off and the rest remains in the grip. Don’t worry, the inside of the top of the chamber has a 5mm hex head and can be screwed out separately. Internal CO2 baffles shown. I just left the baffles out for use with HPA.
Grip frame with all pins removed.
There are 2 knurled pin sizes.
Three 4mm x 20mm. Also found some that worked in a standard size .125 x .500.
Two 3mm x 13 mm
Remove the trigger shoe to allow the trigger to slide out the top.
The stock screw holding the trigger shoe on is a flathead M3 x 6mm
Safety spring and ball are hidden below the pin holding in the sear. Safety bb size is 3mm.
There is a sear adjustment set screw that is accessed through a hole in the front of the grip. Besides the asa threads, these are the only non metric threads on the gun. I think this was a later addition to the gun.
The threads on this set screw are 5/16-24. Use a 5/32 allen key or just the 4mm as its close enough and this should be easy to remove.
Closer picture of sear adjustment screw and spring. Spring has a brass cap on it. I have not been able to find replacements for this. So you might have to get creative if you lose yours. Whatever you might use, just ensure it is no wider than the O.D. of the spring or it won’t allow the sear to travel up and down. One thing I have done is use a 1/4” ball bearing and JB weld it to the top of the spring, which does work. The other thing I have done is used a brass dome solid rivet and cut it to the size and shape I needed, which also works well.
Trigger assembly has a sear release that is spring loaded inside the trigger. If necessary, use a small punch to remove this roll pin.
Pin size is 2mm x 8mm.
I have used alternate springs here just for testing such as detent and safety springs. Seems that as long as the spring pushes the release out far enough that it rests on the pin it works fine. Too stiff of a spring and you will notice a clicking noise when dry firing. That is this little metal piece catching the front of the sear. Not a problem, just kind of annoying.
There are no spares for the little metal sear release. Unless more are in South Africa and get shipped back to PTP. I had some spares CNC’d. So if you need one let me know.
Rainmaker helped get me the CAD file for the replacement part.
Pic of where the air transfer assembly connects to the grip. On some guns the edge of this hole is pretty sharp and you may notice it clip your air transfer assembly oring. Sanding it down a little to take that edge off helps prevent it from clipping the oring when being installed.
You can see the air transfer hole that leads down to the top of the expansion chamber. I have expanded this hole on most of my guns using the next size up drill bit, just to increase airflow a little. Obviously don’t go too big or it won’t seal.
Freak boring
The Armson barrels can be freak bored without any issues. I have had several of them done.
The bolt on the other hand will not work with inserts below .684 in my experience. Therefore it may be necessary to sand down the bolt tip, otherwise the bolt will stick inside of the smaller inserts. I am sure everyone will have a different method for doing this, I don’t have any fancy machine tools so I just basically turned my hand drill into a makeshift lathe and used my Dremel with a sanding wheel to sand down the tip of the bolt. You will need the last 1/4” or so of the tip of the bolt to be sanded down. I would just stop periodically and check it with my .675 insert.
This is also a good opportunity to take some fine grit sandpaper and polish the rest of the bolt.
Semi foregrip mod
I have added foregrips to most of my Semis. I think they are a good addition to the gun and they aren’t a permanent mod, so you can always return to the stock configuration. The foregrip just replaces the front grip screw.
The foregrip I found that works the best and was reasonably priced and easily accessible was one built for an A5 sold by Allen Paintball Products. It also had some milling that kinda matched the gun already.
I bought a handful of the APP A5 grips and took them to the local machine shop to turn down the top to fit in front of the Semi trigger guard. It’s a tight spot that even Automag grips won’t fit into. I was thinking maybe a standard SP gadget grip might work, but there aren’t enough of those floating around. (As of the time of this post, the A5 grip shown on the APP website is not the one shown below, but the one below is the one they have in stock).
The very top should be turned to a diameter of about .675”. That probably needs to extend down about an 1”, but I just had them turn it down to a spot that made sense using the existing milling which ended up closer to 1.3”. Next I drilled and tapped the top to accept M8-1.25 external threaded M5 internally threaded inserts. I bought some 25mm and 30mm M5 set screws to use as my front grip screw. Some of the foregrips were drilled a little deeper than others.
I like to add a 7/8” rubber tip made for furniture to the foregrip to just protect it from getting scuffed up. I didn’t even bother with Loctite as the tension from the set screw in the insert pretty much locks everything in place.
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