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Reply With QuoteInteresting.
Are you using it in a CA carrier? Any fitment issues?
-Chris
I Finally Tried the SD Locking Piece from RCM for Low Powered Rounds
I bought this locking piece from RCM about a year ago under the theory that it would channel more energy back into the bolt and thereby compensate for lower powered ammo. Well I took the SW5 SBR to the range on Saturday (finally) to test this theory out.
I loaded a magazine with about 25 rnds of Remington UMC ammo. Yeah, this is some of the low powered stuff that doesn't usually run reliably in one of these guns. Anyway, I started by shooting about 10 rnds with the standard locking piece. The cases were popping out very weak-like anywhere from 1-4'. I switched over to the SD locking piece and shot the rest. I noticed that they were coming out at least twice as far as with the standard locking piece. I have more testing to do but I thought the initial results looked promising.
I'm not going to promote the shooting of UMC ammo, but I might experiment with loading some rounds a bit lighter than normal to see how the use of the SD locking piece affects it.
I believe that Bobcat Weapons actually used this technique to compensate for resistances of the bolt assembly and rifle hammer springs in their MP5 clones.
Anyway, I found this kind of interesting and will post any additional results that I come up with.
"Sometimes change sucks."- Anne Frank
Reloading: A low impact exercise with a high impact payoff.
Take your wife or GF to the range more. If you want her to get the bug, you've got to expose her to the disease.
Interesting.
Are you using it in a CA carrier? Any fitment issues?
-Chris
Using it in a SW (essentially the same thing, I suppose) carrier and bolt head. The gun was built back in 2000. Not sure exactly when the carrier group was built but I suspect somewhere it that time-frame.Originally Posted by cbecker33
Good to know.
When I had a bunch of RCM parts on the "try before you buy" program I noticed that the RCM LP would fit in the CA carrier, but the CA LP would not fit in the RCM carrier.
I might get a new standard mp5 LP and see how it works when shooting suppressed.
-Chris
"Retro", I know that you are very well versed in the delayed roller blow back system. Did any marks show up on the inside of the receiver? It is my understanding that if the locking piece angle or recoil spring is too weak the bolt carrier will hit the buffer so hard that the rollers will be forced out. If this happens enough the rollers will start to make dimples in the receiver.
In the Frank James MP5 book, he says that you could fire a few rounds without any rollers if you had to, but that would be rough on the gun. I understand that UMC is weak 9mm. My question would be, is it SD weak? My understanding is that the porting in the SD barrel takes 2-300 ft/sec off standard 115 grain ammo. Is UMC that much weaker? If you had a crono and could get a percentage of power between full power 9mm and the weaker UMC. With that ratio it would give you an idea of the difference you would need to make up for to have the same function with the weaker ammo.
Instead of using an expensive locking piece, what about using a weaker recoil spring? If you bought a complete recoil spring unit and a couple of spare springs, I would think you could cut half a coil off the spring at a time. As the spring got weaker, the brass would fly further. If you cut too many coils, cut another spring a little longer than the spring that is too short. I would think that tuning the spring would give you lots of incremental steps. The SD locking piece from the 100 degree locking piece is a big step. This is just a thought. Good luck with your UMC ammo project.
Scott
Attacking the problem from the angle of making the recoil resistance lower will result in less forward forces when chambering a round. So you'll end up with FTF issues. You really really don't want that.
And while I haven't chonographed the rounds yet (that's the more extensive testing that I referred to in the OP), I believe that watching the ejection energy (angle and speed of ejected shell cases) is a great indicator of the amount of energy that is forcing the bolt assy. back. Or at least as good of one as you really need. Remember, IIRC, this energy is usually pressurized in the receiver and exerts more force with the regular locking piece. Using the SD locking piece just redirects it back into the bolt assembly, where you need it. Or at least this is where you need it with weak ammo.
Remember, if the cases are just barely coming out at the minimal force/distance for sustained reliable operation of the gun, there is almost no chance at all that they bolt assy is being forced back against the rear of the receiver (or rather the stock and/or buffer) hard enough to bother anything at all.
Oh, and I think that I only paid about $30 for this locking piece so it's not really very expensive.
ETA: Looks like they cost $35 now.
http://www.rimcountrymfg.com/guns.htm
Last edited by retrodog; 08-04-2009 at 05:37 PM.
All very good points. If there are no marks at the back of the receiver from the rollers pushing out then you have found your answer. As I recall the SD locking piece is 120 degrees and the standard is 100. That is a big shift of the energy into the B/C assembly. My concern was that that might be too big of a redirection of force. The spring reduction of a coil or two would be a much smaller increase of B/C recoil action.
I don't know what the percentage of difference the UMC ammo is compared to say Federal. If reducing the spring by a couple of coils makes the gun run better, I wouldn't think that would adversely effect the ability of the spring to push the bolt through the mag to strip the top round. That's what I meant by if the spring was cut too short. The spring does two functions. It absorbs some of the recoil energy and then uses that energy to return the B/C back to battery and strip the next round. If the spring doesn't have enough power to strip rounds reliably then it is too short. So cut another spring a little longer.
Again, if there are no marks from the rollers at the back of the receiver you have found your fix. I didn't realize the RCM locking pieces were so inexpensive. Thank you for sharing your experiments results.
Scott
Standard LP = 100 deg.
SD LP = 115 deg.
Yeah, the key here to remember is that as far as the recoil spring is concerned, force rearward = force forward multiplied by some friction coefficient. I was working against that back in the early days of experimenting with this gun and trying to get a recoil spring that worked both ways.
So, in essence, you have to exert a definable minimum amount of force rearward to insure you have the near-equivalent forward energy to properly cycle the weapon into battery. Otherwise, using a weaker recoil spring would work fine for weaker ammo. So you are left with needing to find a way to redirect more of the available energy back into the bolt. And I think that the locking piece is the only easy way to do that.
I chronographed a lot of different rounds in my work to come up with a maximum power 147 grain load for running suppressed and maintaining the subsonic speed. During this testing I also checked some of the regular 115 grain ammo that I had been using. I found that there was nearly a 200 fps difference between Fiocchi and S&B ammo. The S&B being the slower. I can definitely feel the difference between the two.
TB has commented in the past (I think he did anyway) that shooting +p ammo in these guns was ok. Obviously any additional force that slams the bolt group back into the rear harder would likely put more wear on the gun but the key is "just how much more wear is caused by this?"
Anyway, I have shot a lot of different types of ammo through this SW5 and my CA89 and CA89K models. I have gotten very used to them and how they feel (recoil) during shooting. The additional force I was getting last Saturday with the SD locking piece and UMC ammo was still notably less than I get with even the S&B 115 grain ammo. I do indeed need to do some more testing but I am in no way even approaching the forces felt in this gun with the S&B or Fiocchi while using the standard locking piece.
Bear in mind, the porting of the SD barrel has the same effect as using these underpowered UMC rounds. This similarity in resulting power between the two is what I based the theory on to try this. I'm definitely going to check inside the receiver though. Thanks for mentioning that.
Mind if I ask what you came up with?
-Chris
It's in one of the other threads. Let me go look.
Done:
You can tell by looking at this that the UMC clocked out similar to the S&B in terms of velocity. Not sure how that is created time-wise and if the S&B causes better cycling due to faster burn rate with more initial pressure. Hmmm, I'm not an expert. Anyway, cycling and extraction/ejection is still going to be a great indicator of force (recoil) to some degree.
Last edited by retrodog; 08-04-2009 at 09:16 PM.
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