Restricting/tuning the velocity of a QB on HPA

[SeanMP]

This is something I was working on last October and posted on another forum. But as there is a similar thread question I decided to repost it here for reference I was trying to tune the velocity of my AR2078 to match a specific pellet preference. Bob suggested fitting a solid transfer port of 1/4" teflon bar stock, rather than using the standard 1/4" white poly tubing, and drilling a hole in it to a size that would restrict the velocity to my desired number and flatten the shot string. I took Bob's idea and went one step further by milling a 2.5mm deep pocket in the valve body using a 1/4" end mill. I then made a series of restrictors out of 1/4" brass rod with various #drill holes in them.
restrictor buttons.jpg

Restrictor Button install in valve body.jpg

If I had only a drill press press to work with. Bob's idea of using the 1/4" teflon bar stock would be the best and would yield the same result. The only reason I chose to go with the brass ports is because I tear my gun down frequently and I didn't want to have to remake a custom transfer port seal once a week.

Here are the before and after results at 1500psi fill

Before restrictorsunrestricted shot string.jpg625fps down to 475fps over 18 shots

With restrictorsRestricted shot string.jpg512fps to 462 fps over 18 shots AVG 498fps

I later found that increasing the fill pressure from 1500psi to just under 1600 flattened the string even more and doubled the shot count.

With the restrictor fitted the performance matches the that of the AR running on C02 caplets. The stock rifle will bounce around either side of 500fps giving a 498fps avg velocity. (which actually impresses the heck out of me that the chinese were able to hold the velocity right at the line)