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Discussion Starter #1
I have a SW5 receiver and kit for a new build and have run into an issue with the mag and trunnion. I've always learned that when you install the trunnion in a CETME/HK it should touch the rails. But on this receiver with the trunnion pressed in all the way the barrel pin holes do not line up and the magazine will not lock up. If I move it forward about 1-2mm then everything lines up and the mag locks up as it should. So is the trunnion touching the rails critical on an MP5?

The mag will only rock into place when the trunnion is forward 1-2mm. If it is touching the rails it is essentially blocking it and there is no lockup. I pulled out the mag release and with the trunnion all the way back and touching the rails the mag inserts fully but the catch is about 1.5mm too high or the magazine slot is low.

I found a post on here that details the an issue with ejection of the last round and it was mentioned by @Shattered Mind that "There was an issue years ago when a batch of receivers apparently didn't have the mag catch hole drilled in the right spot." Whoever did the OP's build just ground out the bottom of the mag catch opening so the catch was canted downward and engaged the mag. I'd rather not do that. One of the possible causes also mentioned was the trunnion was too far back. So that is making me wonder if touching the rails is actually necessary on these receivers. In the pic on the above post it looks like the trunnion isn't touching. Or is the problem with my mag catch hole location?

Would someone with an SW5 receiver happen to have the dimensions for A, B, C, and D? This isn't my receiver but a representation. If I'm going to have to weld and re-drill I want to make sure everything is right.

mp5-hole dimensions.jpg
 

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Yes they should touch. Any gap makes it so the rollers have a crevasse to click into as the bolt cycles. This just adds wear. Typically there are two tack welds at the edges of the rails that connect to the trunnion, and without a good seating this becomes a gap that is hard to weld cleanly. Pictures would help, as for the dimensions it's going to vary quite a bit depending on the manufacturer for A and D. B and C are more standard but the rule of thumb is to take the actual trigger cage and trigger housing you are going to use and space the shelf height ("C"). As for "B", is the hole not drilled yet?
 

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Discussion Starter #3
Here's some pics.

This is the current measurements to the center of the mag catch hole. I've looked at plenty of other receiver pics on the net and this hole seems a little high to me. Maybe its just the difference in the receivers but the .210" measurement looks farther on other builds.

20170421_223854.jpg

This is the trunnion all the way back and touching the rails. You can see the mag catch lip is nowhere near the top of the mag catch cutout. Its also hard to tell from the pick but the barrel pin hole is covered about halfway.

20170421_223912.jpg

This is with the trunnion moved forward about 1-2mm. The mag catch lip is almost even with the top of the mag catch cutout and the mag catch engages once installed. The barrel pin also lines up to where its accessible if need be.

20170421_224035.jpg


To give some more background, this is most of parts kit I bought on GB. It was missing the bolt head, cocking tube support, and sights. The original owner passed away before he could finish it. I'm not sure if he drilled the mag catch hole but the mag lip opening has obvious file/grinding marks. The kit has an original HK barrel and trunnion. I don't know if SW ever manufactured trunnions with their receivers and maybe this is the reason for the discrepancy? Anyway, nothing has been welded but if I have to redo the holes I will. I just want to do it right.

Thanks for the assistance and any tips you can provide.
 

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As for measurements, I am getting .8570" and .2670" so yes the hole seems high. My receiver is also an SW from a flat.

The trunnion doesn't look right seated all the way back, it looks better pushed forward. Also the mag catch and mag position is alot better when it is moved slightly forward. I don't know what to say other than the rails should touch. I've had problems in the past where the flat or finished receiver needs some attention to make the end of the rails parallel to the trunnion, but never such a wide gap. The rollers need to go across that gap as it cycles and it's a potential wear point. Hard to say if it's kosher to have a 1/16" gap. But I have no idea how to fix it other than welding in some metal, which is a PITA.
 

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Discussion Starter #5
I think I figured out the problem....the trunnion. It was mangled in the front somewhat by whomever demilled it. It was cosmetic and I could live with it since the barrel was already pinned. But now I realize that the two nubs that protrude from the back and touch the rails are much shorter than it seems they should be.

Here's where someone caught the front with a cutter.

20170422_093823.jpg

Here's the rear showing the nubs, or lack thereof. Total depth is .0380" on each side.

20170422_093907.jpg

20170422_094025.jpg

20170422_094233.jpg

Compared to new trunnions this one seems to be lacking a lot of meat in the nubs. The edge is not even beveled which seems to indicate this was cut short during the BATF demill and the previous owner cleaned them up.

mp5trunnion-3_1875_detail.jpg

I was hoping to preserve the original trunnion and barrel but now it must go. At least I'll gain another compliance part.
 

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It looks like you found the problem, that is more than half the battle a lot of times. Like you said it sucks that you have to get a new trunnion but at least it will be correct and you will have another 922r part. Please post photos when you get it built.
 
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