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Hk21 FMP parts

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#46 ·
My guess why RCM is considering new oversized rollers is that the HK belt fed guns (and especially the older 21s) seem to really beat up the bolt head due to the cyclic rate/bolt velocity and the recoil spring strength. However I am not sure how they would be different that the plus sized US made rollers already available from HKParts.

Over time you end up with what are essentially dents in the front of the bolt head roller pockets. As the roller dents in the front of the bolt head roller pocket grow it slowly eats away at bolt gap as the bolt head swallows the rollers and the bolt gap they create in conjunction with the locking piece. You can also see wear on the locking piece itself as well that also contributes to shrinking bolt gap. A lot of these surplus and well used kits the bolt head and/or locking piece is at the end of its bolt gap service life.

You can go up in roller size but there is a limit on how big you can go as the pocket in the bolt head is usually somewhere between 8.1mm to 8.15mm. (I am not 100% sure what HK spec is on bolt head roller pocket width). This is why I assumed HKParts has historically topped out at +9 rollers as if you go bigger than that there are good odds the rollers won't physically slide into the bolt head anymore. HK Germany tops at at +4 rollers and you are theoretically not supposed to go bigger than that.... although HKParts has sold greater than +4 US made rollers that I suspect folks must be somewhat successfully using given how long they have been selling them for. I have no clue what the long term ramification are to the gun (if any) using a larger than +4 roller.

I guess you could theoretically maybe go bigger than +9 on a roller dented bolt head if you then used a file to try and open up the roller pocket on the outside of the curved roller dent in the bolt head pocket so a +10 sized (or larger) roller will fit. However, IMHO the best option is really at that point to replace the bolt head and/or locking piece vs. stuffing ever larger rollers into a worn out bolt head so its good that RCM is going to make bolt heads as well.

Once you run out of bolt gap on a HK belt fed you can't push the barrel from the trunnion, spin and repin either so you are limited to going up in roller size or replacing the bolt head and/or locking piece. In my limited experience its always been the bolt head that wears out first and that seems to align with my conversations with some of the belt fed builders as well. Although I have heard of folks in the past welding up and recutting the HK beltfed barrel locking lugs to try and push bolt gap back into spec by having the barrel lock essentially lock back farther in the trunnion, mechanically mimicking a traditional barrel repin operation.
 
Discussion starter · #51 ·
jbntex said:
My guess why RCM is considering new oversized rollers is that the HK belt fed guns (and especially the older 21s) seem to really beat up the bolt head due to the cyclic rate/bolt velocity and the recoil spring strength. However I am not sure how they would be different that the plus sized US made rollers already available from HKParts.

Over time you end up with what are essentially dents in the front of the bolt head roller pockets. As the roller dents in the front of the bolt head roller pocket grow it slowly eats away at bolt gap as the bolt head swallows the rollers and the bolt gap they create in conjunction with the locking piece. You can also see wear on the locking piece itself as well that also contributes to shrinking bolt gap. A lot of these surplus and well used kits the bolt head and/or locking piece is at the end of its bolt gap service life.

You can go up in roller size but there is a limit on how big you can go as the pocket in the bolt head is usually somewhere between 8.1mm to 8.15mm. (I am not 100% sure what HK spec is on bolt head roller pocket width). This is why I assumed HKParts has historically topped out at +9 rollers as if you go bigger than that there are good odds the rollers won't physically slide into the bolt head anymore. HK Germany tops at at +4 rollers and you are theoretically not supposed to go bigger than that.... although HKParts has sold greater than +4 US made rollers that I suspect folks must be somewhat successfully using given how long they have been selling them for. I have no clue what the long term ramification are to the gun (if any) using a larger than +4 roller.

I guess you could theoretically maybe go bigger than +9 on a roller dented bolt head if you then used a file to try and open up the roller pocket on the outside of the curved roller dent in the bolt head pocket so a +10 sized (or larger) roller will fit. However, IMHO the best option is really at that point to replace the bolt head and/or locking piece vs. stuffing ever larger rollers into a worn out bolt head so its good that RCM is going to make bolt heads as well.

Once you run out of bolt gap on a HK belt fed you can't push the barrel from the trunnion, spin and repin either so you are limited to going up in roller size or replacing the bolt head and/or locking piece. In my limited experience its always been the bolt head that wears out first and that seems to align with my conversations with some of the belt fed builders as well. Although I have heard of folks in the past welding up and recutting the HK beltfed barrel locking lugs to try and push bolt gap back into spec by having the barrel lock essentially lock back farther in the trunnion, mechanically mimicking a traditional barrel repin operation.
Click to expand...
My parts kit came with a genuine HK stamped bolt head, makes me wonder if it was a replacement part or just manufactured with some hk parts here and there.
 
#54 ·
jbntex said:
My guess why RCM is considering new oversized rollers is that the HK belt fed guns (and especially the older 21s) seem to really beat up the bolt head due to the cyclic rate/bolt velocity and the recoil spring strength. However I am not sure how they would be different that the plus sized US made rollers already available from HKParts.
snip
Click to expand...
I love your post not gay

I've never worn out a German bolt head like that, before I crack them at the corners. Clones yes, they are never heat treated correctly. They always fail at the corners
 
#56 ·
barry.n.bunch said:
I am not a gunsmith or machinist but regarding the roller pocket recess, what about building a replaceable pocket recess like is used in the SIG AMT?
Click to expand...
there is no material on earth that thin, that could stand that abuse.

The problem is, in the real work you replace the part, because its a wear part. Its that simple. The only reason this is a question is people cant due the proper maint due to import laws
 
#57 ·
Glynch said:
My parts kit came with a genuine HK stamped bolt head, makes me wonder if it was a replacement part or just manufactured with some hk parts here and there.
Click to expand...
Just recently I spoke to a friend of mine that lives in South Africa.
Many years ago he served in the Rhodesian Army, and was involved with the fielding of the FMP G3 rifles and the assessment of the HK-21's (which they did not adopt)
His memory was that every new production FMP G3 and HK21 that he handled had HK marked bolts and carriers.
His thoughts are that as part of the licensing agreement HK provided the bolts and carriers.
I suppose that is a good way to ensure that FMP didn't deliver any new guns without paying the royalties since HK would know how many bolts and carriers they had supplied to FMP!
 
#58 · (Edited)
TaylorWSO said:
I love your post not gay

I've never worn out a German bolt head like that, before I crack them at the corners. Clones yes, they are never heat treated correctly. They always fail at the corners
Click to expand...

I see the same wear and failure pattern on the German bolt heads as well but maybe its just my bad luck.

It also seems like they eat up some bolt gap early on and once the dent gets to a certain point the damage/wear seems to slows down as the roller has more and more material to try and displace as the roller forms an ever expanding dent to the front of the pocket and the bolt head starts to almost cradle the round roller. Just my casual observation anyway. I have never cracked either type of bolt head German or US made, but have cracked the E extractors through the pivot pin hole.

I actually try to never drop the bolt on an empty chamber on the HK belt feds out of concern for further wearing the bolt head or locking piece and similar to my M60 run a snap cap on the end of the belt to prevent bolt damage. With a belt/round in the gun it significantly slows the bolt velocity down upon closure as it advances the belt and strips a round from the link and chambers it. I noticed that you could slap the charging handle on dry gun and at times it would slam the bolt home so hard that the action would overcome the locking lever and bounce open a bit. Given how expensive and difficult these parts are to get now a days and maybe its futile effort but I figure a snap cap is cheap insurance to try and prolong the bolt head and locking pieces life. I also let the bolt down slowly vs slapping the charging handle on an empty gun as well.

This is one of my German 21E bolt heads where you can see the distinctive roller "dent" forming in the front of the pocket.

Image
 
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