Excessive Bolt Gap in Cetme/G3 Does What?

[Drakejake]

Moving on to another question about the roller-delayed blowback action of the Cetme .308 and several H&K firearms (and this is a fundamental question): Assuming that bolt gap determines (or indicates) the length of delay between firing and the unlocking of the bolt assembly (timing), does excessive bolt gap INCREASE the delay so that the rifle might not cycle, or does it REDUCE the delay so that the bolt assembly rebounds too soon and too forcefully, thus causing case head separations and excessive wear on the rifle? Even on this basic question, there is conflict between people who seem to be well-informed. This conflict seems to imply how difficult it is to understand this system. But note that in the thread cited below, citations and authorities are asked for and given.

http://hkpro.com/forum/showthread.php?t=51320&highlight=bolt+gap

Drakejake

 

[texlurch]

From my experience, low gap delays the action and slows extraction, unless you are sitting on zero and the rollers are loose in the trunnion.

Excessive gap puts the rollers very high in the recess in the trunnion, which allows the gun to unlock too soon, before pressure is bled off the chamber. Then the bolt carrier hits the buffer too hard, and will sometimes allow the rollers to pop out and create dimples at the rear of the receiver.

My CETME with a .004 gap recoils like a bolt action, and fails to extract with any repeatability. This Century built gun has some issues, as adding a new LP and +4 rollers DECREASED the gap. The trunnion, barrel and bolt don't appear to be excessively worn, so the barrel is off somewhere (press).

My G3 with an .014 gap shoots smoother than an AK, with less recoil. Extraction pitches the spent shell 15-20 ft. It is the definition of sweet.

If you study a cut-away of the CETME/HK chamber, trunnion, LP and bolt, you'll see how pushing the bolt farther away moves the rollers up the ramp in the trunnion, and out of the scallops in the LP; reducing the mechanical leverage required to unlock the bolt.

At least that is how mine work and appear to me.

 

[rbernie]

Can we first please agree that the bolt GAP does not control anything, and is simply a means of instrumenting the relationship between the locking piece and the bolt head and rollers and carrier and such? Please?

From the other thread on HKPRO:

Zero bolt gap is not going to slow down the action, it will speed it up. With zero gap the rollers are no longer firmly pinched between the locking piece and trunion. They are floating loose and because the bolt isn't resting on them, it is resting against the carrier. So when the gun is fired the first few millimeters of movement are straight blowback, which is too fast for the HK bolt/carrier mass. Then there is a slowing down when the rollers slam into the trunnion and start working, but now they're being slammed into locking piece and all sorts of wear and strange timing issues come up.

I can readily agree with this, if the zero bolt gap precludes the proper extension of the rollers when the bolt is otherwise as far forward as it can go. I can also see where, under very lucky circumstances, zero bolt gap could result in full extension of the rollers into the recesses and no such reduction in unlock force would result.

If you study a cut-away of the CETME/HK chamber, trunnion, LP and bolt, you'll see how pushing the bolt farther away moves the rollers up the ramp in the trunnion, and out of the scallops in the LP; reducing the mechanical leverage required to unlock the bolt.

Well, I agree with the premise but I'm not sure that I can see the conclusion. Yes, moving the bolt further forward pushes the rollers onto a slightly tapered surface in the trunnion cutout, and I agree that this taper would push the rollers into the bolt head slightly. But how can the carrier move rearward BEFORE the bolt head, which would seem to be required in order to achieve unlocking without the rollers rolling back into the deepest section of the recesses?

I was of the impression that the tapered trunnion recess was there to ensure that the bolt could always travel as far forward as possible (headspace properly) and self-correct its timing a bit (easy at first, then stiffer, then unlock) to keep the overall cycle within a nominal time/force window.

 

[rbernie]

Another thought:

I was of the impression that the bolt head contacted the breech end of the barrel as a/the means by which it ceased forward motion. Certainly, examination of my CETMEs would seem to indicate this type of contact is occuring (e.g. that the bolt's 'in battery' location is in part dictated by the breech end of the bolt), at least when the chamber is empty. (I don't know if the roller locking system allows the headspace to 'float' when there is a round in the chamber, since the bolt never really locks up hard anyway. It would be interesting to have this confirmed/denied..)

Presuming this to be correct behavior, and presuming a designated distance (with tolerances) between the locking recesses and the rear of the barrel, it would seem that a larger bolt gap cannot push the bolt further forward. The breech end of the barrel would by design precludes further forward motion, implying that the increased gap can only serve to hold the carrier and locking piece itself back slightly. The only way that an increasing bolt gap would seem to be able to alter the location of the rollers in the trunnion when the bolt is in battery would seem to be if the breech end of the barrel allows the bolt head to move forward further than it would normally sit relative to the locking recesses. Does this make sense?

 

[texlurch]

Presuming this to be correct behavior, and presuming a designated distance (with tolerances) between the locking recesses and the rear of the barrel, it would seem that a larger bolt gap cannot push the bolt further forward.

Exactly. The bolt stops on the barrel, then the LP pushes the roller out, into the locking recess in the trunnion.

So, if the barrel is too far into the trunnion (towards the rear of the gun), the rollers will stop sooner, which stops the LP further towards the nose, and gives more gap, between the bolt and carrier.

So from my reasoning, you now have the rollers sitting more towards the point on the LP, and farther along the ramp in the trunnion, closer to the bolt. This gives the bolt more leverage against the roller, less distance for them to travel to unlock, a "weaker" overall hold on the bolt.
In an extreme case, the bolt may just be sitting there, with only the recoil spring to hold against chamber pressure.
So instead of the system delaying and bleeding off some chamber pressure, it is able to pop open early and come flying back, smacking into the buffer and stock.

 

[texlurch]

Here is a diagram of a CETME chamber, locked. Only way I know to increase bolt gap, is either larger rollers, or press barrel into trunnion (towards stock, or to the right in the pic).

Now, if barrel moves to the right, then the bolt goes with it, correct?
This moves the rollers up the ramps in the trunnion, deeper into the bolt, farther towards the front (point) of the LP. This pushes the LP and carrier back, away from the bolt, increasing bolt gap (air). Think about the leverage required, to make the rollers roll/slide up that ramp, against the LP, to push the LP to the rear and unlock the bolt, allowing the mass to recoil.

Now, visually move the rollers back, up the ramps, into the bolt and towards the point on the LP. Think about the distance, friction and leverage against the rollers. Does it make sense it will unlock easier and move out of the chamber quicker?

If you have two of them, with proper clearance on the cocking handle, the one with more gap is much easier to cock than the one with a tight gap. This is because you have more leverage against the bolt to unlock it. It acts exactly the same when it is fired.

 

[ceetee]

The bolt gap does not CONTROL, or DETERMINE anything.

The bolt gap is just an indicator of when the moving parts get so worn that they need attention/replacement. That's why grinding the bolt is a bad idea. With the bolt ground, you have nothing to tell you (reliably) when all the other moving parts get too worn.

 

[texlurch]

Yes, it is an indicator of the condition of the locking parts of your gun.

The question was, if you have a high gap, what does that indicate, and what could the results be if fired, in that condition?

Pretty much "everyone" understands why grinding the bolt, to give a false gap, is not "kewl". But that would be the "other" thread...