Steadily Decreasing FPS

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Steadily Decreasing FPS

My friend had a DBoys M4 CQB-R. He replaced its brand new spring with a M110 spring to satisfy CQB requirements. The first time, the chrono reported 365 FPS. The last time he went to the field, the chrono reported 315 FPS (fifth time at the field). Last night, he went again and chrono reported 305 FPS. It seems the AEG drop by 10 FPS every time he plays.

I had similar experience. First time, my APS ASR106 Mini Patriot M4 chrono'ed at 330 FPS. The fifth time, it chronoed at 260FPS. That means I lost about 14 FPS per game.

We are somewhat baffled at this steady loss. Can it be the spring losing tension over time? Or the air seal opening bit by bit?

Jake
Sat, 05 Oct 2013 01:45:06 +0400

Even folks at Evike agreed that FPS will drop over time. A friend of mine purchased a Cyma AK47 this weekend. It chronoed 380 FPS, 15 more FPS than allowed at our local indoor CQB field. The sales person told him to fire off 5000 rounds before we show up at the field.

Jake
Wed, 16 Oct 2013 08:43:33 +0400

Found the culprit in my case. It is the air seal opening up a little bit at a time, not the spring tension. After taking the gearbox apart, I found that the piston head o-ring did not seal well with the custom ported brass cylinder. Apparently the brass cylinder has expanded over time with the piston impact, because the exact same o-ring sealed well with the original ported stainless steel cylinder. After putting the gearbox back, only replacing the cylinder, the gun chrono'ed at 317 FPS.

One lesson learned is to use stainless steel to reduce cylinder maintenance. Another lesson learned is to replace o-rings whenever FPS drops.

Jake
Sat, 19 Oct 2013 05:24:53 +0400

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