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OH, USA
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Posted: 2/6/2024 9:28:19 PM EDT
So I have an AP5-P SBR. I typically will shoot with my rugged obsidian 45 on the 3 lug mount in the short configuration. I shoot anything from 115gr to 147 gr subs.
I have been contemplating changing out the locking piece. I know this subject is pretty beat to death across forums, but I just can't seem to find a clear answer on it. Some guys say not to bother, some say go to an 80*, some say go to 100*, etc, etc. I have noticed with my rifle that it is not terribly quiet compared to the suppressor on a pistol. I don't have another MP5 to compare it too in terms of impulse. Should I order the 80* as that seems to be commonly recommended for a suppressed SBR? Or since it's a 45 suppressor with likely lower back pressure, should I go with the 100* or stick with the stock 120*? |
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GA, USA
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Posted: 2/6/2024 9:36:38 PM EDT
[#1]
In my K size, I'm running rim county 80 degree locking pieces. They run fine with factory 115, 147, and my own handload 147gr. All with and without a suppressor.
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Posted: 2/6/2024 11:15:24 PM EDT
[#2]
@xoldsmugglerx
Is your gun set-up as a PDW? |
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OH, USA
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Posted: 2/6/2024 11:16:54 PM EDT
[#3]
Quote HistoryOriginally Posted By osprey21: @xoldsmugglerx Is your gun set-up as a PDW? Yes, I have the Choate folding mp5k stock on it and usually shoot it suppressed with my obsidian 45 suppressor |
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Posted: 2/7/2024 4:24:50 AM EDT
[#4]
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USA
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Posted: 2/7/2024 5:55:26 AM EDT
[#5]
I believe the consensus is to go with the 80 degree piece for the K model. Logic being you want to delay the unlock so less energy is transferred into the shorter receiver, thus reducing wear and possibly bulging at the back of the firearm.
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USA
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Posted: 2/7/2024 7:09:21 AM EDT
[#6]
Many run the 80 (and some minimize bolt gap too) in our full size models in order to minimize port noise & maximize suppression. I use the 80 in my SP5 and -6 rollers (as I couldn’t find -8 at the time) which puts mine in the middle of the bolt gap range. It still functions 100% with 115, 1000fps 147, and 900fps 147 which is everything I use.
My SP5…  9mm subsonic ammo suppressed comparison Amphibian’s MP5…  CT9KS Suppressor Pew does too for their testbed… https://pewscience.com/sound-signature-reviews-free/sss-6-85-research-supplement-unsuppressed-subguns |
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SC, USA
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Posted: 2/7/2024 7:48:08 AM EDT
[#7]
I can tell you that we broke the hammer on my buddies AP-5 running the standard LP with a Ti-Rant 45 can, probably no more than 200 rds. fired. He's got a #28 LP now!
Sad part is I have a #28 LP in my spare parts stash and didn't even think to use it. 
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FL, USA
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Posted: 2/7/2024 8:06:04 AM EDT
[#8]
Picture below I documented the cyclic rates:
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Posted: 2/7/2024 2:13:35 PM EDT
[Last Edit: lil_Sig]
[#9]
xoldsmugglerx,
Do you notice any difference in the impulse when you fire suppressed? If it is noticeably faster / harder cycling? If so you should definitely try an 80* locking piece. If you do install one be sure to check your bolt gap with it. You can get the RCM for a reasonable price. So I say give it a try. If it gaps we'll and cycles reliability with the ammo you use. By all means the slower the unlock / cycle rate the better. As long as it cycles and ejects reliably. It really is one of those things you just have to check out on each individual gun. |
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OH, USA
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Posted: 2/8/2024 1:50:40 AM EDT
[#10]
Quote HistoryOriginally Posted By lil_Sig: xoldsmugglerx, Do you notice any difference in the impulse when you fire suppressed? If it is noticeably faster / harder cycling? If so you should definitely try an 80* locking piece. If you do install one be sure to check your bolt gap with it. You can get the RCM for a reasonable price. So I say give it a try. If it gaps we'll and cycles reliability with the ammo you use. By all means the slower the unlock / cycle rate the better. As long as it cycles and ejects reliably. It really is one of those things you just have to check out on each individual gun. I would say it's not very noticeable in terms of impulse change from suppressed to unsuppressed, but I didn't really focus on that the last time I was at the range. I'll have to go back and really pay attention to it. I think I'll order the RCM 80* and try it out after checking the bolt gap. Do you think I should order + size rollers in addition just in case bolt gap is bad? |
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OH, USA
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Posted: 2/8/2024 1:55:54 AM EDT
[#11]
Quote HistoryOriginally Posted By Clicker58: I can tell you that we broke the hammer on my buddies AP-5 running the standard LP with a Ti-Rant 45 can, probably no more than 200 rds. fired. He's got a #28 LP now! Sad part is I have a #28 LP in my spare parts stash and didn't even think to use it. Damn, that's what I'm worried about. A lot of guys on HKPro and reddit say don't change a thing because it's semi auto. I disagree with that idea since running F/A has little to do with this. Thinking from a physics perspective, the bolt will reciprocate at a similar speed and force weather running in S/A or F/A. Thus, it can do the same damage regardless of F/A or S/A.
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OH, USA
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Posted: 2/8/2024 1:57:16 AM EDT
[#12]
Quote HistoryOriginally Posted By amphibian: Picture below I documented the cyclic rates: https://c3junkie.com/wp-content/uploads/2023/09/MEAN-CMMG-MP5.jpg Good to know, I'll try the 80* for sure. The only thing I'm worried about is reliability in a K-sized rifle. I'll just have to run it through its paces to be sure. |
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USA
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Posted: 2/8/2024 7:14:30 AM EDT
[#13]
Quote HistoryOriginally Posted By xoldsmugglerx: Do you think I should order + size rollers in addition just in case bolt gap is bad? No because you won’t know what size you’ll need. In my case, as posted above I needed -6 rollers to bring it to the middle of the bolt gap range. |
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Posted: 2/8/2024 8:07:08 AM EDT
[#14]
Quote HistoryOriginally Posted By CJofFL: No because you won’t know what size you’ll need. In my case, as posted above I needed -6 rollers to bring it to the middle of the bolt gap range. for best practices suppressed don't you want to aim for the largest in-spec gap you can achieve? My full size spits out powder particles hard enough to sting shooting left handed. |
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USA
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Posted: 2/8/2024 8:51:54 AM EDT
[#15]
Quote HistoryOriginally Posted By BlackbirdRider: for best practices suppressed don't you want to aim for the largest in-spec gap you can achieve? My full size spits out powder particles hard enough to sting shooting left handed. No, exactly the opposite, one wants a minimum in spec gap for the longest delay. |
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Posted: 2/8/2024 11:31:56 AM EDT
[#16]
Quote HistoryOriginally Posted By CJofFL: No, exactly the opposite, one wants a minimum in spec gap for the longest delay. Quote HistoryOriginally Posted By CJofFL: Originally Posted By BlackbirdRider: for best practices suppressed don't you want to aim for the largest in-spec gap you can achieve? My full size spits out powder particles hard enough to sting shooting left handed. No, exactly the opposite, one wants a minimum in spec gap for the longest delay. This. If you take your bolt and carrier out. Notice the more the bolt is positioned closer to the carrier. IE the less bolt gap the more the rollers protrude outward. The more they protrude outward. The more they set into the trunnion roller recess. |
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IN, USA
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Posted: 2/8/2024 5:32:56 PM EDT
[#17]
I'm currently waiting on an RCM 80 degree locking piece for my full size Zenith MKE. Unsuppressed it runs 100% with all weights with the factory 100 degree LP. When I attach my Tirant 45 it'll have a hiccup about once or twice per mag. Extracts just fine but runs the new round up into the chamber at 60-70 degree angle and causes a dent in the case and significant bullet setback. When I attach my R9 it jams every single round in the same fashion after firing the first round loaded from the mag by dropping the CH. It's obvious to me it's not a mag issue or ammo issue. It's a backpressure issue causing the bolt to unlock too fast and cycling the bolt too fast. I was looking inside the receiver the other day and seen a very small shiny spot just where the roller rides. I've gotta slow down my cyclic rate if I want to shoot it suppressed. I bought the R9 specifically to live on this SBR
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Posted: 2/9/2024 4:48:07 PM EDT
[#18]
Quote HistoryOriginally Posted By lil_Sig: This. If you take your bolt and carrier out. Notice the more the bolt is positioned closer to the carrier. IE the less bolt gap the more the rollers protrude outward. The more they protrude outward. The more they set into the trunnion roller recess. Quote HistoryOriginally Posted By lil_Sig: Originally Posted By CJofFL: Originally Posted By BlackbirdRider: for best practices suppressed don't you want to aim for the largest in-spec gap you can achieve? My full size spits out powder particles hard enough to sting shooting left handed. No, exactly the opposite, one wants a minimum in spec gap for the longest delay. This. If you take your bolt and carrier out. Notice the more the bolt is positioned closer to the carrier. IE the less bolt gap the more the rollers protrude outward. The more they protrude outward. The more they set into the trunnion roller recess. I watched a video on measuring the gap, and I am still learning this platform. The upshot of it was HK's are factory set to the largest gap, clones might be anywhere in between. As the weapon wears, the gap closes til it eventually touches the bolt carrier and becomes a straight blow back. So the trick is actually to attempt to maintain at the smallest spec setting for the softest recoil and least port blowback? I picked up an 80º but have not checked the gap yet. |
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MI, USA
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Posted: 2/9/2024 5:13:08 PM EDT
[#19]
I put a 100deg RCM locking piece in my AP5P at the same time I got my suppressor and have put about 2k through it since. Mostly suppressed and about 1k of that has been 147 and 150gr. I keep a close eye on the inside of the receiver for the start of roller dents, as well as the buffer for excessive wear. Zero signs of anything and it looks like the bcg barely kisses the buffer. Recoil is noticeably softer with the 100deg as well. It runs everything just fine.
The 80deg is most recommended obviously but reports of reliability are all over the place. Some it works suppressed and unsuppressed, some it wont run only 115gr without a can and for others it wont run 115gr with a can. My buddy recently picked up a 90deg and so far it has been gtg for everything, whereas the 80deg didn't work for him unsuppressed. I would guess that success with the 80deg has a lot to do with bolt gap and how broken in the gun is. A well broken in gun with bolt gap on the higher end might have the best luck. No way I'd run the stock locking piece suppressed on the AP5P though. RCM locking pieces aren't very expensive, and well worth experimenting with IMO. |
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Posted: 2/10/2024 7:23:25 AM EDT
[#20]
I went with the RCM 80* lp in my PTR 9KT.
Had to change the rollers to get the bolt gap back in spec but it runs fine with/without a suppressor, all types of ammo. I shoot it with a brace attached. Not FA, but I sometimes run a binary trigger and would rather be safe than sorry. |
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USA
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Posted: 2/10/2024 8:51:29 AM EDT
[#21]
Quote HistoryOriginally Posted By BlackbirdRider: As the weapon wears, the gap closes til it eventually touches the bolt carrier and becomes a straight blow back. No, if the gap were so large that the rollers didn’t recess into the trunnion, it would be straight blowback, although I’m not sure how that’s possible. |
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Posted: 2/10/2024 10:11:37 AM EDT
[Last Edit: BlackbirdRider]
[#22]
Quote HistoryOriginally Posted By CJofFL: No, if the gap were so large that the rollers didn’t recess into the trunnion, it would be straight blowback, although I’m not sure how that’s possible. I am referring to this video by James Williamson..starts around 11:30  Teufelshund Tactical Heckler and Koch Bolt Gap Review |
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USA
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Posted: 2/10/2024 4:55:28 PM EDT
[#23]
I don’t want to say he’s mistaken as most consider him an expert, but if there were zero gap, the rollers would still be recessed into the trunnion, therefore a delay. On the other hand, if the rollers aren’t recessed into the trunnion (maximum bolt gap) there wouldn’t be a delay (straight blowback). Perhaps he’s alluding to; if there was zero bolt gap and too much play between the rollers and the trunnion recesses, there wouldn’t be a delay until the rollers slammed into the face of the recesses and the delay may be overcome by that inertia.
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MI, USA
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Posted: 2/10/2024 6:47:02 PM EDT
[Last Edit: Redshift556]
[#24]
Quote HistoryOriginally Posted By CJofFL: I don’t want to say he’s mistaken as most consider him an expert, but if there were zero gap, the rollers would still be recessed into the trunnion, therefore a delay. On the other hand, if the rollers aren’t recessed into the trunnion (maximum bolt gap) there wouldn’t be a delay (straight blowback). Perhaps he’s alluding to; if there was zero bolt gap and too much play between the rollers and the trunnion recesses, there wouldn’t be a delay until the rollers slammed into the face of the recesses and the delay may be overcome by that inertia. I believe he is mistaken and not alluding to a zero gap scenario because he also mentions that too much bolt gap would cause too much of a delay. The rollers should be fully recessed into the trunnion regardless of bolt gap. This is my understanding of how it works. Force is pushing back against the bolt when the gun is fired. The rollers are preventing the bolt from moving because they're locked into the trunnion. The locking recesses in the trunnion are angled so that the rearward force is pushing the rollers back into the bolt. The rollers are essentially squeezing the angled parts of locking piece when this happens. As the locking piece is squeezed it is pushed back. The deeper the locking piece(lower the bolt gap), the further back the locking piece has to be pushed until rollers are able to go back into the bolt and the system unlocks. This would increase lockup time. A higher bolt gap means that the rollers don't have to push the locking piece back as far and allows the system to unlock sooner. |
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IN, USA
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Posted: 3/16/2024 5:52:42 PM EDT
[#25]
Quote HistoryOriginally Posted By kbi: https://hkparts.net/all-parts/rcm-hk-mp5k-n-pdw-28-locking-piece-80-degree/ FYI I got the notification that these came back in stock here and grabbed me one. If anyone has been waiting for one nows your chance. |
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NH, USA
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Posted: 3/17/2024 10:46:27 PM EDT
[#26]
If there is no bolt gap, then the force of the shell pushing on the bolt can transfer to the carrier. Like using the cocking tube support to push back on the carrier, the bolt unlocks very easily. That is why the bolt gap needs to be between 0.010" and 0.018".
Scott |
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PA, USA
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Posted: 3/19/2024 2:04:45 PM EDT
[#27]
Just went through this with my AP5-P. It runs 100% unsuppressed with any ammo with factory locking piece. I then added an 80* locking piece to shoot suppressed , checked bolt gap and it’s same as with factory looking piece. It runs 100% with 147 subsonic and my octane 9. If I switch suppressors and run my YHM R9 I will get some failures and weaker extraction, switch back to octane 9 and no issues.
My conclusion is the octane has more back pressure than the R9 so the 80* works perfect. The R9 with the factory locking piece runs 100% with no signs excessive wear or bolt moving to fast. I was told that an 80* locking piece was not always needed when running suppressed, depending on the suppressor used. Your gun will tell you if it’s required or not. |
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Posted: 3/21/2024 11:56:37 AM EDT
[#28]
So people are putting a #28 locking piece in their full size MP5 suppressed? Am I reading that right?
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PA, USA
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Posted: 3/21/2024 2:14:07 PM EDT
[#29]
Quote HistoryOriginally Posted By Blacktoothgrin: So people are putting a #28 locking piece in their full size MP5 suppressed? Am I reading that right? Full size don’t need it, just the K models. |
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FL, USA
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Posted: 3/21/2024 6:54:42 PM EDT
[#30]
Quote HistoryOriginally Posted By arjohnson: Full size don't need it, just the K models. Quote HistoryOriginally Posted By arjohnson: Originally Posted By Blacktoothgrin: So people are putting a #28 locking piece in their full size MP5 suppressed? Am I reading that right? Full size don't need it, just the K models. |
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PA, USA
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Posted: 3/21/2024 8:01:46 PM EDT
[#31]
Quote HistoryOriginally Posted By amphibian: Yes, I do.... as I posted above, I am getting 88 RPM slower cyclic rate in full auto doing so. Needed? Nope....but this hobby has nothing to do with need. I want a slower full auto cyclic rate and doing this works. Not much more you can do to tune an MP5. Works fine for me unsuppressed as well. You’re using it by choice, not a necessity. The K models need it to prevent wear and damage which can happen. |
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FL, USA
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Posted: 3/22/2024 4:02:36 AM EDT
[#32]
Quote HistoryOriginally Posted By arjohnson: You're using it by choice, not a necessity. The K models need it to prevent wear and damage which can happen. |
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Posted: 3/22/2024 12:19:11 PM EDT
[Last Edit: Blacktoothgrin]
[#33]
So I can use the #28LP in my full size. What are the benefits besides lower RPM? Reduced port pop? less blowback suppressed?
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FL, USA
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Posted: 3/22/2024 8:43:44 PM EDT
[#34]
Quote HistoryOriginally Posted By Blacktoothgrin: So I can use the #28LP in my full size. What are the benefits besides lower RPM? Reduced port pop? less blowback suppressed? I haven't dug into all his research to know why is doing that as well. I presume he has all the meters to give him data on that stuff. |
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Posted: 3/23/2024 2:23:59 PM EDT
[#35]
Thanks
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FL, USA
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Posted: 3/23/2024 4:43:32 PM EDT
[#36]
Also @CJofFL was telling me about the JP-5 locking pieces and they are interchangeable with the MP5 as seen on their site:
https://www.jprifles.com/buy.php?item=JP5-LP Notice that they also make a 60 and 70 degree in addition to the 80 degree. It is obviously a different gun that shares MP5 parts compatibility but I wouldn't be surprised if the 60 or 70 work in some MP5's under certain conditions. |
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Posted: 3/23/2024 4:54:21 PM EDT
[#37]
Can I assume if I swap out a locking piece to a diffrrent degree, say 110⁰ to an 80⁰, I'll need to also do corrections on rollers?
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FL, USA
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Posted: 3/23/2024 7:12:31 PM EDT
[#38]
Quote HistoryOriginally Posted By aod886: Can I assume if I swap out a locking piece to a diffrrent degree, say 110 to an 80 , I'll need to also do corrections on rollers? |
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