I took out the 21 since its been a awhile-man that thing has some recoil compared to the 23 (I was shooting off the bench but it still was rocking)

It runs fine with  a 45 degree LP in it (at .013-.014 gap), it barely runs with a 40 degree piece in it.

(ETA it will run found out it was the trigger pack acting up)

would putting the 40 degree piece in it (or maybe a 36) but increase the gap to the .018 range make it run smoother (less recoil)?

would just increasing the gap with the 45 run smoother?

see updated info in the reply

If by smoother you mean less recoil then the bolt has to stay closed longer, allowing more gas to escape out the barrel before the bolt opens.  Increasing the gap allows it to open sooner by shortening the delay.

I don't know enough about a 21 to give specific advice but my understanding is it should run normally with a 40 LPand be able to cycle a 36LP (since it's  optional for a 21).

It sounds like you might be able to do some tweaking of parts (lighter locking pawl spring?) To get a 40LP to work but you are then in a state of diminishing returns.

I run a 40LP in a 24" barreled Cetme.

Overall on HK roller-locked systems you have two “knobs” you can adjust to modify the timing of the bolt unlocking from the trunnion.

The primary way the unlock timing is modified is via the locking pieces.    The greater the locking piece angle rating the easier it is for the bolt carrier to unlock from the trunnion.   So the greater the angle of the locking piece you put into the gun the easier it will be for the bolt to unlock from the trunnion.  This is why the short barrel variants of the same rifle caliber take a higher angle locking piece (i.e. the HK53 takes a 65 degree where the 33 takes a 55 degree)  and the larger the rifle caliber (given the same barrel length) the lower the locking piece angle.  (i.e. HKG3 takes a 45 degree but a HK33 takes a 55 or a 21e takes a 40 degree vs. a 23e that takes a 70 degree)

The locking piece is the primary control knob on bolt unlock timing.     In theory, you should also be able to change locking pieces without the bolt gap changing.  However my experience in practice is that changing the locking piece usually does result in a bolt gap change.

The other option for changing bolt unlock timing is the bolt gap via the roller size.  The greater the bolt gap the more pre-unlocked the bolt is inside the trunnion, the easier it is for the cartridge to push the bolt from the trunnion and fully unlock the action.

Other items that can make a change in recoil and ROF are the recoil spring and the bolt lock spring.

To answer  your specific questions.

“Would putting the 40 degree piece in it (or maybe a 36) but increase the gap to the .018 range make it run smoother (less recoil)?”

If it barely runs with a 40 degree piece I suspect that it won’t run well with 36 degree LP which is even more difficult for the gun to unlock the bolt.

Increasing the bolt gap on a given locking piece doesn’t reduce recoil.   You didn’t say what the bolt gap was with the 40 degree locking piece installed.  But let’s assume it’s the same as the 45 degree LP at ~0.014.   If you put +2 rollers in that will bring your gap up to  0.018 which will mean the bolt is that much more pre-unlocked so the bolt carrier will come back harder.  However, given you mention that the gun barely runs with a 40 degree locking piece installed there shouldn’t really be a lot of recoil to mitigate right now.

However if you have functional issue with the 40 degree LP (FTEs) and need more oomph to get it to run reliably and/or get the ROF less “choppy” than you could put larger rollers in to get it a bit more power into the recoil stroke.   That said its important understand where the bolt gap is with the 40 degree LP installed before making a recommendation for different rollers.

“Would just increasing the gap with the 45 run smoother?”

If you have high amount recoil with a 45 degree LP at 0.014 than increasing the gap to 0.018 is going to further up the recoil.  Greater bolt gap = more pre unlocked bolt head = carrier unlocking earlier/faster = carrier hitting the back of the receiver harder.  The only time this equation doesn’t really apply is once the bolt gap shrinks to zero.   Given you are right in the sweet spot of spec bolt gap spec on the 45 degree locking piece I wouldn’t recommend trying to shrink the bolt gap via minus rollers just to try and tame some of the recoil.

A 21E should run without issue on a #9 / 40 degree locking piece. It would be interesting to understand where the bolt gap is with the 40 degree locking piece and whats the origin of the #9 / 40 degree piece (German? RCM? MM?)

Thanks for the replys, timely thread as I just back from the range with the new rollers.

what I thought was a "recoil" problem turned out to be a crap trigger pack-that has been replaced. The trigger pack was beat to death and was causing problems.

The gun I have is a TSC 21, not a "real 21e" but it has the 21e spring/stock etc.

45 vs 40 degree. The 40 degree is noticeably smoother. I tried both back to back and their is a lot less recoil on the 40. Less sharp.

bolt gap, I changed it from .005 all the way up to .020" with both locking pieces,  If there was a difference you couldn't tell.

When I used 8mm rollers the bolt gap changed form .018 with the 40 and .015 with the 45, as best I could tell

I ordered a 36 degree LP, and I will try it out.I think it will work seeing as the 40 still throws brass into next century.

BTW ZQI ammo is shit, that would barely run the gun

Your experience with modifying bolt gap to measurably change the way a gun runs (when the bolt gap is within service spec) is similar to mine.

I sort of look at locking pieces like a adjustment knob that makes changes in rounded numerical increments.  Changing the bolt gap on an existing locking piece anywhere between roughly 0.008 and 0.02 is like making changes in decimal point increments.   Changing from a 40 to a 45 degree LP is like moving from a 1.0 to a 2.0.  Moving the bolt gap on a 40 degree LP from 0.014 to 0.018 is like moving from 1 to 1.2.

Sometimes when the results you want fall in between available locking piece angles, moving bolt gap is one of your other fine tune options.  (along with modifying the recoil spring and/or locking lever spring)

Obviously with all of the hardware changes you can make at the host level, ammo grain weight, how hotly loaded, and if a suppressor is in use come into play as well.   That is why HK has so many different locking pieces for the same host gun to deal with a myraid of different ammo types out there and if the host is suppressed or not.

If you have a 21 but are running the 21E recoil system (buffer and spring) that you can certainly give the #17 LP a try.  It should shave off some additional RPM and recoil in the process given the shallower angle vs. the #9.

Glad to hear its running properly with the #9 LP now as unless you were running some really weak ammo there was probably something else wrong with the gun  (which it sounds like there was and you got patched up)