Buffer weight, carrier weight and adjustable gas. Two questions.

[cas]

Okay, two separate issues with two rifles:

Gun #1 is a "race gun" type setup with adjustable gas. (16", mid length gas) There isn't a problem with this rifle in any setup, I was just fiddling with things to experiment and learn, and came across something that seemed odd and I wanted bounce it off someone else to see what they thought.

Each gas adjustment was the standard turn it down, then open it till the bolt locks back method

Started with a standard bolt/carrier/rifle buffer.

The first change was to a lightened carrier, for which I turned the gas was turned down some.

Next step was to a lighter buffer for which the gas was cut even more.

Followed by an even lighter buffer, again the gas was reduced.

The last step was to an even lighter carrier... but that required that the gas be opened up considerably.

That's the one one that's confusing me. Why would it suddenly need a good deal more gas to lock back? Less inertia?

Any ideas/theories?

Rifle #2 is actually a problem. I have a little retro CAR-15 that today suddenly decided it doesn't like 55gr ammo. Heavier stuff cycles and ejects fine, but 55gr ammo won't make it out of the port. With rounds in the mag I get a feed jam, usually with the empty caught between the outer bolt face and the rear underside

of the charging handle. If I shoot it with an empty mag, the empty case ends up laying on the follower.

The only thing that was changed between last time working fine and this time not, was that I installed a JP trigger/hammer springs. The original ones were kind of heavy but... could that minimal a difference suddenly be making it cycle too fast? Maybe it was right on the verge before and now the minor change was just enough? Weird. I have an H buffer I will try in it next time to see if that solves the problem.

[ken hebert]

first impression: gas port, block, or tube may be partially occluded, or the ferrule end of the tube where it meets the carrier key may be worn and you are getting some bleed, loss of pressure.

[cas]

first impression: gas port, block, or tube may be partially occluded or the ferrule end of the tube where it meets the carrier key may be worn and you are getting some bleed, loss of pressure.

I dunno, these aren't changes over a lot of time, for the most part this is just swapping parts one minute to the next.

[ken hebert]

wait, edit. i apologize, i just kinda skimmed through your original post and didnt catch that it was another rifle having the bolt over problem. my bad.

#2: might the lighter springs have decreased lock time?

Edited May 2, 2011 by ken hebert

[mpeltier]

Problem 1 is most likly the shorter than rifle length system is unlocking too soon (letting gas escape) with the lightest components and there is not enough gas in the system to have enough inertia to overcome the spring pressure. The shorter gas systems ara a bit more sensitve to this due to an already reduced volume of gas, by adjusting the gas (reducing) you are reducing the volume further. A little more weight (to delay the bolt unlocking) or more gas (Increases inertia directed to the BC group) will fix the problem.

Problem 2 may indeed be helped with the H buffer. Extractor/spring may also be the culprit as the CAR seems to run a bit hotter temp in the bolt and springs fail a little sooner in them.

[mpeltier]

wait, edit. i apologize, i just kinda skimmed through your original post and didnt catch that it was another rifle having the bolt over problem. my bad.

#2: might the lighter springs have decreased lock time?

Ken,

Lock time, as far as I know, is the time from pressing the trigger, hammer falls and firing pin hits primer and "BOOM". Are you refering to an increased cyclic rate, which is the time it takes for the rifle to fire and BC reciprocates and chambers another round ready to shoot again?

Also the lightened hammer by JP with the JP spring would decrease lock time by virtue of its lighter hammer, not the springs. A millspec hammer with lighter springs would typically result in increased (longer) locktime.

Not pickin on ya ken, just seems a lot of confusion sometimes over definitions and description of the operating systems. Any real Gunsmiths out there have any corrections/comments to add???

[dchang0]

Gun #1:

Have you checked the gas key for leakage on the very lightest carrier, the one that takes a lot of gas to lock back?

In terms of classical mechanics in physics, it is most likely that the momentum of the lightest BCG+buffer combo you have tried is having trouble overcoming the spring force of the buffer spring. I suspect that you went past the optimal weight of BCG+buffer to match the buffer spring.

You'll have to use a weaker buffer spring to reduce the gas again. BUT, there is a big caveat. You need the buffer spring to be strong enough to strip rounds out of the magazine AND also to slam the BCG closed hard enough to lock the bolt. That presents a "floor" past which you cannot go.

Edited May 6, 2011 by dchang0

[cas]

It seems that gun #2 has "dwell issues" and isn't related to springs, buffer etc. H buffer didn't change anything. Have to decide which way to go from here.