So I got both my VMs (one magnum and one tiger stripe) in shooting shape a few weeks back, and have spent some time getting them to shoot in the 280 to 300 fps range. Here is my experience so far...
Both VMs had the valve set to 12oclock and the bolt choke screw set wide open. New main spring in each, and running a shorty armson barrel on the magnum and a lapco adapter with a cocker bull barrel on the tiger stripe. HPA tanks were outputting around 850 psi.
- initial velocity on the tiger stripe was around 320 fps. Magnum was around 280.
- was a bit puzzled with the difference, guns had the same guts. Took the magnum apart to check the airlines and pathways to make sure nothing was interfering with airflow, all good.
- remembered that I had put fresh correct-sized orings on the bolt in the magnum. Swapped those out for some that were a bit less tight, that helped a bit.
- put the armson barrel on the tiger stripe VM, suddenly it was shooting below 300. Light bulb moment.
- got a freak barrel and sized the insert to the paint I was using. Put it on the tiger stripe VM in place of the armson. Velocity went to 340!
- tried to dial it down with the bolt choke screw. Had minimal effect, I think I was able to drop it by around 20 fps so still shooting hot.
So long story short, on the magnum I ended up rotating the main valve so that I was on the second largest port with the bolt choke screw wide open. This gets about 290 fps with the 850 psi hpa tank. On the tiger stripe VM I used the same valve setting but choked up the bolt and was able to get 275-280 fps with the 750 psi tank. Interestingly, depending on which port is being used, opening up the choke screw in the bolt can actually decrease velocity... my theory is that since the air passages in the gun and valve so small (and when the choke screw is fully choked up, there is still a gap in the bolt air path that is close to the size of the actual air passage in the marker/valve) if the burst of high pressure air from the valve is allowed to expand before it hits the ball (ie, you open up the choke screw at the back of the bolt) then it reduces how much energy is transferred into the ball at the bolt face.
My takeaways to this point:
- The main driver of velocity in the VM is the valve, and the second largest port on the valve should provide enough juice to get a ball going at field velocity (280 to 300 fps).
- The bolt choke screw slightly reduced velocity when using the largest valve port. All other ports, velocity increased if the bolt was choked up.
- HPA tank output pressure will have an impact, 750 psi and above should work.
- Paint to barrel match is way more important than I originally thought, I saw a difference of 40 fps between a large bore barrel and a freak barrel with a fairly snug fit (although not underbored as I would do with a pump setup).
Both VMs had the valve set to 12oclock and the bolt choke screw set wide open. New main spring in each, and running a shorty armson barrel on the magnum and a lapco adapter with a cocker bull barrel on the tiger stripe. HPA tanks were outputting around 850 psi.
- initial velocity on the tiger stripe was around 320 fps. Magnum was around 280.
- was a bit puzzled with the difference, guns had the same guts. Took the magnum apart to check the airlines and pathways to make sure nothing was interfering with airflow, all good.
- remembered that I had put fresh correct-sized orings on the bolt in the magnum. Swapped those out for some that were a bit less tight, that helped a bit.
- put the armson barrel on the tiger stripe VM, suddenly it was shooting below 300. Light bulb moment.
- got a freak barrel and sized the insert to the paint I was using. Put it on the tiger stripe VM in place of the armson. Velocity went to 340!
- tried to dial it down with the bolt choke screw. Had minimal effect, I think I was able to drop it by around 20 fps so still shooting hot.
So long story short, on the magnum I ended up rotating the main valve so that I was on the second largest port with the bolt choke screw wide open. This gets about 290 fps with the 850 psi hpa tank. On the tiger stripe VM I used the same valve setting but choked up the bolt and was able to get 275-280 fps with the 750 psi tank. Interestingly, depending on which port is being used, opening up the choke screw in the bolt can actually decrease velocity... my theory is that since the air passages in the gun and valve so small (and when the choke screw is fully choked up, there is still a gap in the bolt air path that is close to the size of the actual air passage in the marker/valve) if the burst of high pressure air from the valve is allowed to expand before it hits the ball (ie, you open up the choke screw at the back of the bolt) then it reduces how much energy is transferred into the ball at the bolt face.
My takeaways to this point:
- The main driver of velocity in the VM is the valve, and the second largest port on the valve should provide enough juice to get a ball going at field velocity (280 to 300 fps).
- The bolt choke screw slightly reduced velocity when using the largest valve port. All other ports, velocity increased if the bolt was choked up.
- HPA tank output pressure will have an impact, 750 psi and above should work.
- Paint to barrel match is way more important than I originally thought, I saw a difference of 40 fps between a large bore barrel and a freak barrel with a fairly snug fit (although not underbored as I would do with a pump setup).
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