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I've seen this discussed a few times on this board and other and strong arguments being made for both. I have not, however, seen any actual 91's damaged from putting a can on them.

I have a stock HK91 with a flash suppressor on it. I want to mount a surefire 338 can onto it.

Do I need to put the '18' locker onto it?
 

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Check out the guide to locking pieces, which is what I am guessing you call a "locker" at: Guide to HK Locking Pieces and The Roller Locking System in Technical Detail


L-R: HK91/G3/SR9 (no marking 45°), G3K marked 18 (50°), PSG1 marked 19 (40°), MSG90 marked 23T-T is for Titanium (40°) and MSG90 30T for different ammo pressure. (40°)

The 18 (for shorter barreled G3K) has a higher anger of 50° than the standard G3 locking piece (no marking 45°) and thus the 18 unlocks easier than the standard g3 locking piece. When you suppress a rifle w/o any other changes (i.e. like porting the barrel in the MP5SD) you tend to generate a higher recoil impulse at the bolt and thus if you change anything one should go in the opposite direction (i.e. lower angle locking piece) to make it harder to unlock.

Thus if one tried anything would go with a 19, however, need to check to make sure there is nothing different on the PSG1 locking piece to bolt setup.

This poster is talking about using a 17 (?): do hk rifles suppress well? - AR15.Com Archive

More reading on suppressing a G3/HK91 (just because I am interested):
Suppressing the HK91
Suppressed HK91
do hk rifles suppress well? - INGunOwners

http://youtu.be/YM7w-ds2M7Q
 

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Not to disrespect but the locking piece guide is 100% useless to our questions. Moat of it is for pure educational purposes and not a guide on what LP to use or to start with. The other links are great, as with anything I do the best way to find out is to spend the time and money myself.....
 

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Not to disrespect but the locking piece guide is 100% useless to our questions. Moat of it is for pure educational purposes and not a guide on what LP to use or to start with. The other links are great, as with anything I do the best way to find out is to spend the time and money myself.....
Have to disagree and go further to state that appears you are actually wrong in thinking the locking piece guide, or at least some of the information contained with in, is 100% useless to the question (then again is easier to debate 100% useless as one only has to show the tiniest use for the locking piece guide and then it is only 99.995% useless ;-).

My primary use and reason for the locking piece guide is a starting point and definition, so everyone knows what angle is for a standard G3 locking piece and what the angles are for other listed locking pieces. Figured with someone having their first post here, calling the locking piece a locker, that is was a good thing for everyone to have a starting point on what the locking piece is, what angles they are etc.

Next use is to have a ready guide of a few common angles and what the the numbers are, while one can always consider a custom grind and hardening, getting an off the shelf locking piece tends to be positive.

Lastly there does not appear to be a nice clear "use this locking piece for that load and suppressor, thus one maybe have to make parallels with systems (i.e. MP5K-PDW, HK53 etc) and see what type change in the locking piece angles are needed in these cases and then estimate from there for the start of testing.

Now it appears the "other half" of the project team has showed up with just under a thousand posts, a lot of the above information may already be known and not of use, however, easy to recyle the data back to the bits ;-). Take care, good luck and agree you have to do testing for nearly anything, was just trying to get a discussion started and help a newby by the post count.
 

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I can see your view but those guides didn't help me any. Then again there's only three LPs I would consider and I already knew about those. I went from
A #18 LP and a .3mm gap to a #17 LP and zero gap. Then to +4 rollers and only brought it up to .2mm. I'm going to test fire my 51 with this and if still doesn't help, no suppressor for this gun. It just sucks to come here and get the same answers. "they don't suppress well", I never asked if it did it "well". I wanted to "just cause"......
 

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We are discussing several different but interconnected issues here, bolt gap, which locking piece to use with a suppressor and suppressing various barrel lengths of roller locked firearms.

- Any time you change the locking piece, even between locking pieces of the same number your bolt gap may change. The same is also true for the bolt head, gun/barrel/trunnion and roller sets. Thus in your case it appears the #17 LP was either ground differently and/or has more wear than the #18 LP (G3K). Possible if you tried a different locking piece the already ok but at the low end bolt gap of your HK51 would not drop so much. At times have see US made supper over size rollers listed for sale of +6 and +8 (currently out of stock at HK91 G3 Rollers and Retainer for HK 91 HK21 PTR91 V51 CETME, HK +2 +4 Roller, RTG Gun Parts )

- Is the #17 LP originally intended for use in a HK21 or similar belt gun and if so what is the angle on the #17 LP? Reason I ask is that I shoot one of Mike's MM23eK's and he sells custom locking pieces for the the shorter barrels in 223 and 308, thus there maybe be a few more options for you to try plus if anyone is shooting a HK21/HK23e/HK21e/MM23eK/MM21eK etc you might be able to "borrow" one to try .... myself not for sure which locking pieces fit in which other guns as there is some differences in the extractor etc. Just a passing tough.

- With the suppressor on wonder if the standard unmarked G3 suppressor would even allow your HK51 to function ... probably not but at $12 is a low cost option and one should be able to find someone with a g3/HK91 locking piece to try.

- The initial poster was suppressing an HK91, which was assuming is the full size and nominally has a barrel length of just under 18", however, see in your discussion that you are working with a HK51. Are you and the first poster connected in this project (you commented on "our question" which could just be a similar line or reasoning)?

- Lastly you did not get the same answer here of "don't suppress well", you got data points and a few possible useful or not useful references showing the initial question of suppressing an HK91 with a 18" barrel was possible. You did not get a complete and fully tested answer and are left to on your own at that point, in particular as the project(s) now include a very short barrel 308, the HK51 .... which shall we say will be more interesting with the limited bullet stability and very impressive muzzle blast (i.e. if the discussion would have started with the HK51 might not have replied and waited for others ;-).

Take care, good luck and hope to see you posting on your testing results, good, bad or marginal.
 

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Don't forget can design as well some generate significant back pressure vs others. I run surefires on my g3 clones and psg1-k gun
 
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