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.