Effects of reciprocating mass on accuracy? AR specific

[Tim K]

I have a new upper from JP. Another rifle I own from them gives excellent accuracy. I ordered this upper with their newish adjustable mass BCG and an H2 equivalent SCS. The goal was ultimate reliability. All total, the reciprocating mass of the new rifle is 6 oz heavier than that of my other JP.

I've spent a ton of time trying to work up a load for this new upper, and I just can't break 1 minute. Shooting the same Ammo in both guns, the new upper consistency gives groups about twice the size of the old rifle.

Out of frustration, I swapped the heavy BCG and SCS for the low mass pieces from the other gun. It was a very brief test, not at all conclusive, but it appeared that accuracy was going to be much better with the low mass components.

It seems counterintuitive to me. Heavy stuff ought to delay unlocking a few milliseconds, getting the bullet farther past the gas port before things start moving, if they do actually start moving before the bullet exits the barrel. The barrel is 14.5", mid length gas.

Anyone got a plausible explanation for why reducing reciprocating mass should improve accuracy?

[XC700116]

DISCLAIMER: I don't really know the nuance of harmonics in a gas gun specifically to give the following as a definitive answer, therefore the use of anecdotal evidence and the use of the beginning qualifier of "I would assume"

I would assume it has to be some sort of induced/different harmonics and that is effecting the accuracy node/exit timing. I've seen gas guns go from hammers to not so much based on adjustments to a gas block that is in turn changing how things at the back end are running. Same with a couple that will or won't shoot suppressed.

The other thought is that it may require a different technique in driving the rifle due to it's slower operational cycle. ie heavier or lighter load on the bipod, ect.

[Erni]

I think I would start by swapping the new buffer into the known good rifle and load combo to see if the impact is negative and the good gun turns into a bullet hose. If it does then something is up with the buffer. If it does not, go through the new rifle and see if something is off.

The only thing relavant to buffers in my brain is this:
This guy did a bunch of tuning and testing on buffers, and tried for a buffer weight which did not bounce around but I dont recall if he studied accuracy.
https://vuurwapenblog.com/?s=Buffer&submit=Search

[TFOGGER]

I'm wondering if the slower unlock time with the heavier reciprocating mass may transmit more recoil force to the rifle chassis sooner(because the BCG isn't moving independent of the receiver), in effect starting the movement of the barrel/chassis before the bullet has exited the muzzle?

[CS1983]

Wouldn't the heavier stuff have an effect on the 3 pulse recoil of a gas gun, not just unlocking? I'd think that would require a different method of driving it.

I'd give JP a call and ask their opinion, too.

[davsel]

Did you swap bolts?
Just a thought.

[Great-Kazoo]

I'm here for the solution. You consider calling JP, next year to see what they say / suggest ?

[Hoser]

Strange. The textbook solution is a heavy assed carrier.

But the results on paper are all that matter.

[Irving]

I'm wondering if the slower unlock time with the heavier reciprocating mass may transmit more recoil force to the rifle chassis sooner(because the BCG isn't moving independent of the receiver), in effect starting the movement of the barrel/chassis before the bullet has exited the muzzle?

I was thinking exactly this, but since I hardly know anything about how rifles work, wasn't going to say anything. Makes sense to me though.

[MrPrena]

All Approx Number just to compare.
Rate of fire‎: ‎700?950 round/min cyclic (11.67 to 15 hz)
So it travels about 0.086s to 0.067s to a cycle dep on mass of bolt and velocity of ammo.



- 0.35" bolt carrier movement for bolt to unlock,
plus 1.750" case length
2.15" to eject a case;
to pick up a new round, 2.75"
for last round lock-back, 3.2".

5.56mm NATO M855
Vmuz=922 m/s (3,025 ft/s or 36,300 in/s)


l=14.5" barrel
v=(36,300 in/s)
14.5in / 36,300 in/s

approx 4.00x10^-4 sec for bullet to clear out the barrel
0.4 mili sec


0.086s to 0.067s to a cycle to pick up a new round, 2.75"
How about approx how long would it take 0.35"?

0.067s/2.75in = time bolt open/.35"

(0.067s/2.75in) (.35in)= time bolt open
8.53x10^-3s =
or 8.53mili second=

therefore bullet would leave the barrel even way before bolt rotates.
Oscil of metal from explosion is more significant than bolt travel time.