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MP5 Build Questions (Pressing Barrel, Pinning Barrel, Front Sight Tower)

13K views 12 replies 8 participants last post by  Extremevx 
#1 ·
Hey, so I just got done with rolling my LSC MP5 flat complete with the weldment set, cocking tube, paddle release and trunnion. I used an original new HK trunnion and have a new RCM Navy barrel. I have the rest of the parts to finish my build, I just have a few questions regarding pressing in the barrel, pinning it and installing the front sight tower.

I know that the headspacing needs to be between .010 and .018, but the MP5 manual is not so specific regarding the pinning of the barrel. I am fairly new to drilling and welding, but I definitely take my time to ensure that I am doing it properly. I have a drill press and TIG welder in my shop. I am seeing mixed answers on previous forum threads regarding these questions:

1. Do I need to press in my barrel at around .010 - .018 and then pin the barrel or do I leave some extra room and then pin?
2. Which drill bit do I need to drill the new barrel with? I see a few members saying you need a #9 carbide, and others saying 3/16". I also have the barrel pin jig kit I got from hkparts if that helps.
3. Regarding the front sight tower, I have the RCM one and need to know what's the right way of installing it? Do I just hammer it down and then align it with the rear sight? The barrel has ridges on it but I am not sure if they hold the front sight in place or are just there for looks. I see some saying I need to epoxy the front sight tower but I am not sure if that's the right way of doing it. I am not looking to put the sling mount on the tower, but if that is required then I guess I'll need help with that as well.

Any help would be much appreciated! Will post photos once complete, building it into a pistol.
 
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#11 ·
First of all, my understanding is that the smith is not legally supposed to possess an unmarked receiver. You are supposed to build and possess the firearm yourself. If you could hand carry it in and stood there maybe, the smith would work on it. His FFL would be in jeopardy if there was an inspection if he had an unmarked receiver. Since it was unmarked, how would he enter in in his book, as a rolled and welded flat is a firearm? Did you tell the smith that you would be shipping an unmarked receiver to him? I know for myself I would not put my whole business at risk for a customer's couple of hundred dollar item.

I would think the tool listed on GB would be easier to get a drill bit closer to the work, while the HK Part tool might be better for the actual pressing of the pin. To me, cutting the barrel from one side to the other, staying within the trunnion holes would be the real trick. Going from both sides would be next to impossible to end up with one true hole. There is also the issue of the RCM barrel being nitrided. I would think that a square tool would be needed to go through the high carbon layer and not walk the tool off center of the trunnion. Then a drill bit to drill on the squared off barrel. That tool to square off the barrel would need to be short so there would be very little, if any deflection as it squares the round barrel.

It is my understanding that that most builders press the barrel to 0.020" bolt gap. This is a very critical measurement. The barrel will move forward from the pounding of the bolt group and "seat" the barrel into the pin. So the bolt gap will diminish a couple of thousands after the gun has been used. With a new barrel, I'd think you'd want the high end of the bolt gap. If the barrel is pinned at 0.010" BG, after the barrel "seats", the BG will be out of spec, after a little use.

I have a friend that built his own "K" gun. He said that he used a 5mm tool to use with the 5mm pin for the press fit. He used a 12 ton press. He said that it took a couple of times to press the barrel because of the "spring" of the press frame. The barrel would load up and move too much so the BG would go from too much to too small when the barrel moved in the trunnion. He found that using a small hammer and gently tapping the trunnion while pressing made for smaller smoother pressing barrel movement.

My understanding is the front sight must be pressed on after the barrel is pressed and pinned into the trunnion. Then like the barrel in the trunnion, the "triple tree" is pinned to the barrel with a rivet. If the gun was dropped the front sight won't move. If you don't want to use the front sling, but there should be a rivet to hold the front sight in place on the barrel.

Disclaimer, I have not built a roller locked gun from scratch. This is my understanding as to the recommended procedures. Someday I plan to build one myself, but to date I have not built a roller locked from a flat. Jeff Walters has had "Build Weekends" at his shop. He has been very free with information and tips on building roller locked guns. My understanding is he is not so good with typing, but is more than willing to talk on the phone and share his experience. I would highly suggest that you call him. He is a wealth of information. YMMV.

Scott
 
#7 ·
using your barrel pin jig will square it up on the press when it comes time to push the pin in so yes that will help. If you try to use a drill press the drill bit will wonder off, because your trying to take the meat off the side of the barrel, thats why you need a mill.
 
#8 ·
After months of working and finishing my 94 type receiver, I found a nifty tool for sale on GB that would've solved most of the issues I had. So for any noobs like me out there, I would recommend to check this out:
AC Weapons HK Builders Tool Vise Rod Barrel Press : HK Parts at GunBroker.com

Note to admin: not trying to advertise this for sale and I have no affiliation with this seller.
 
#9 ·
Bigrig- Quick question for you regarding the barrel pinning. How were you able to secure and perfectly align your trunnion when drilling for the new pin? Were you able to use your barrel pin jig from HK parts to do it with? How did it turn out? I am concerned once I correctly press my new barrel my hole for the new pin will not be straight.

Thanks!
 
#10 ·
Sir: I went down this same road a few years back, what I did was:
1) Press barrel in to place to achieve proper bolt gap
2) Started with a 3/16 X 1-1/8 FL 4F SE Carbide end mill to cut starting slot through the new RMC barrel.
Please note end mills of this type do not take well to side loads, be careful or it will snap.
3) Followed with a 5mm STR SH STR FL chuck reamer to bring the hole to full size
4) Used a rivet gun (not the cheap air hammer type) with a insert and gently drove the pin in.
I also use this rivet gun insert to remove the pins.
The end mill, reamer I purchase from ENCO tool, don't know if the part numbers are still good but:
End mill 79413688
Carb Chuck reamer 79891271

As far as installing the front sight tower I made an alum bushing that fits inside the cocking
tube and lines up the tower and tap it home.

I don't know how to post pictures, but if you would like to me to email you pictures just send me a note.

Take your time, good luck!

Chet
 
#13 ·
Excuse my ignorance, but I have to ask. If the barrel moves forward under the pounding from the bolt due to the sloppy fiit of the pin, would the gap between the trunnion and carrier increase?
Since the gap we are really measuring her is not the bolt to barrel gap as most people think, ala. Headspace. What we are really measuring is the preload on the rollers expressed in a dimension between the carrier and trunnion. If the carrier was touching the trunnion, we could say the is no load on the rolder and the bolt would effectively rattle around and jump back across the gap when firing.
 
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