So theres some bending and welding involved. Do the kits out there make it pretty easy to bend a flat? I could understand it not being "right" and that being a issue.

The welding, well I mean it doesn't take a ton of skill to weld something together. The skill part really comes in on just making it look nice.

The barrel is pressed into a trunion, and drilled and pinned. So its like an AK, again nothing too difficult.

If you have a mig welder, drill press, and a hydraulic press, it looks like its not that complicated. But Ive been deceived before.

I mean is it really that hard?

Tactical Pursuit has a video series on Youtube where he builds a clone.

It is my understanding that the difficulty in building MP5 clones is in the minutia. You have a very tiny window for error when it comes to things like shelf height and position, which tends to be the achilles heel with semi builds. Mess up by a few millimeters and you have a non functional gun.

Had a friend start down this road and never got his HK33 clone running.

It's hard enough that for some 20+ years, the number of builders that could be trusted to reliably deliver a 100% functioning weapon could be counted on one hand. 

You need squared receiver rails, the correct receiver geometry to accept a stock, a top that is level for the rear sight base, a properly aligned barrel and reunion, a correct shelf height and angle, a straight triple tree, proper magazine catch height...the list goes on.

When Military manufacturers like POF and MKE (who have cranked these out by  hundreds of thousands) are still putting out the occasional turd, that's a pretty good indication of the difficulty level.

"Now kids, don't try this at home..."

I've done it with a 100% receiver.  You will need a TIG welder, a MIG will just put to much weld down.  I have not bent a receiver though.  Maybe one day I'll try a 53, but it's not really even on the list now.  I am a machinist, so for me, it's just another job.

Jason

Yes, it's very difficult. It is very easy for your bends (and almost everything else) to not be quite right, even if you're extremely careful.

They really need to be TIG welded. Even with TIG, warping is a real problem unless you're very careful. You want to minimize the heat buildup. A copper heat sink should be used when welding the cocking tube. The spacing has to be correct (very slight gap). That's a relatively simple aspect, but contractors for Century Arms welded many with incorrect gaps.

The bending jigs available are not the same style HK (or an HK-licensed facility) would utilize in mass production. They have giant presses made to do just one job. The jigs can work, but everything has to be just right. When it becomes not right, you must first be able to recognize this, then stop and reposition before continuing again.

When you're done forming and welding the receiver, it has to be machined correctly if you want correct optics mounts. I'm not sure that can be done without a mill. I'm sure many who roll flats omit the optics mount area. It's easier to weld a picatinny section to the top. I wanted guns that were like the factory models though.

I've never formed an HK receiver from a flat. I did the research and concluded it's way better to buy a finished receiver. I've welded cocking tubes and trunions into place and built guns from kits. That's much simpler than forming the receiver.

If you have a barrel/trunion combo already in proper headspace (like from back when you could get a kit with a live original barrel) you can remove what's left of the receiver and weld the trunion into the new receiver shell, so no press is needed. That's how I built my G3.

I think a PTR91 receiver (without cocking tube) was $175 back then. My FMP G3A3 kit with barrel was $250. I finished it at gunsmith school ten years ago.

I purchased my MP5 and HK53 clones complete because that was the least expensive option. MP5 kits dried up when they started selling for more money as post-86 dealer samples around 2004. They're still worth more that way today. This happened because HK changed their sales policies to cut out SOTs.

I was looking at it doing it, until I calculated the cost of tooling up...even before I calculated the cost of screwing up a few flats learning how not to do it. Way cheaper to buy a Zenith (or three).

If you want to do it for fun / skills / whatever, rock on.

It's not as hard to get one built as it is to make one look nice.

Besides the bending and straightening mandrel tools I have my favorite one is a brass rod I had turned down that holds the receiver and cocking tube in line. Stepped down half way to fit tight in the receiver and tube, then made a plug that keeps the tube and the front sight lined up when you weld the trunnion in.

Im currently building one. I have some notes to share.

I made my own bending jig from some bars and steel that i was allowed to take from the scrap bin at work. The very second i stood back to look at it after welding, i immediately saw abetter way to make it. my point on the jig is its far cheaper to make it yourself, than to buy one. and i do believe mine is correct.

Do not drill the lower front pin hole.

I cant stress enough about the TIG welder. I personally did use a cheap flux core welder from harbor freight, cause its what i had. With it set to minimum power and about3.5 on the wire speed, it seems to have done the job. where the seams are, i made "spot welds" i guess. right after welding those spots have a nice little hump on both side of the receiver. Easy to file flat. I did these all along the seam. little spot welds in a line then file flat. only time will tell if its good enough.

I was more than willing to buy it pieces at a time and spread it out. I personally cant stand to sit still and always have a project to keep me busy. that being said, you will be worlds ahead to save and buy a parts kit so you have everything all at once. I simply had to have parts right now so bought parts as i was ready for them. This will not be safe queen so i bought new and used parts. I have rcm, pof, and other parts in this gun. I have no issues with it having no collector value whatsoever.

Buy the LSC flat and weldment set. after comparing the cheaper one used and a friend built on an LSC, it is superior.

If you want this thing engraved at all, send it out before you bend the flat. Its easier for the engraver, and you dont have to worry about shipping a firearm.

some of the g3 parts will work on an mp5. the big one is the lower. There are deals to be had on true mp5 lowers but you can always use a g3 with the second tang hole cut off then clip and pin it. Change out the extractor for the mp5. There are varying opinions on the hammer spring. 65 shipped with a coupon from sportsmans guide a couple months ago for the g3 lower, and a 25 dollar extractor from rtg.

I bought a used barrel off gunbroker. It was improperly removed from the trunnion. I got some time on a lathe and trued it up. It seems to be fine

the people on the hk forums are quite friendly and i rented a truing mandrel set. I may have taken my time or got lucky, but it proved mostly unnecessary but cheap insurance.

To sum this up, i will be building another. It is one of the most challenging gun related projects i have done. I have also learned alot along the way. Get a tig welder. Ive spent more time on dressing welds than anything else

After watching gunbroker for months, from time to time a complete parts kit can be had for less than 1k. Looks to be about a once a month occurence.

Like i said, only time will tell if this thing is up to snuff, its a toy for me, and something to tinker on. I realize there is no flow or structure to this post, but if you have any questions ill answer what i can.

Building one is not hard, building a squared and functioning one is where it becomes tedious. TIG is the only acceptable way to weld a receiver, demilling the cocking tube is a PITA if building from a parts kit. LSC flat is the way to go for flats (don't use the weld set that comes with them, it is trash) properly fitting the mag catch is also tedious and should be done by hand. Turning down brass rods for heat sinks for welding saves you from a lot of headaches too. If you decided to spend the money on tooling and have any questions you can PM me or post in here as some guys on here can help you out.

It's not at all hard to build one providing you take each step in the correct order and pay close attention in the barrel pressing phase and setting the cocking tube gap.

I have built two G3's.  They both run, but there are cosmetic issues.  To do it right you need a TIG welder and a milling machine.  I faked it with a flux core MIG welder and a drill press, but it was not clean.

To say it isn't very difficult is just inaccurate. One can be really good at it, but it's still hard.

RTG Parts sells a mandrel set

Straightening mandrel set

Man if this thing wasn't 1800 bucks Id take a gamble on building it

Having a kit with an intact barrel makes it significantly easier.

Well I am moving forward with making one

Gunna start off with a CETME kit, but with a little twist.

This will atleast get me going on HK type gun builds which I think will be fun

$1800 I'd chop up my MP5 clone pistol if someone offered me that

Lol

Germans parts and mp5/40  kit

Anyhow i got a cetme L kit. Everything turns out okay I'm building mp5s next year

Buliding 9x weapons isn't that hard, depends on mechanical ability. I started over a year ago from nothing and have built the PSG91, SR91 with 20' german poly barrel now threaded and nitride, 2x 91, 91k, 93, 53-300, 5, 5k. All shoot lights out. The only hang up I had with any of them was the 5k, turned out to be a bad case deflector, now fixed. Once your setup, there's nothing better. From start to finish, building to Parkerizing.

Tig welding is a far better weld application than mig. I bought a portable 120 tig and set it up according to wyatt swaim's setup for 1/16th gage steel. After about an 2 hours I was off to the races. All of the rifles and subs look great too, nice and straight. I never had any issues with the semi shelf, I pinch it to stay put then tack it, double check the measurement and then make it permanent. People make it out to be harder than what it really is. You can get by with outstanding results with minimal equipment. I don't need to use heat sinks when welding. I had a couple bars turned down by a cnc shop to be used for alignment rods, namely for the cocking tubes. long on one end for the 91 and short on the other for the 5k.

Having a mandrel helps a lot, got that from rtg and works in all 9x guns. Using a laser boresight is key when setting up the sights, double check and then double check again. You can absolutely turn out rifles and subs just as good as hk on your own and don't let anyone tell you different, it's all up to you. The only hard part is making the decision to make the jump. I can't tell you how wonderful it is to just buy parts and put it all together and refinish it yourself. Also made the jump to invest in a Quincy and a Grizzly to blast and park. You can leave it parked or paint on top, it's up to you and the quality is all up to you and not dependent on someone else's skill level. I'm not bragging, just trying to say that it's not impossible to do. There isn't some great mystery to it.

It doesn't seem like it should be that hard to build an MP5...

but here we are


There is a reason why skilled people like Mike at TSC will only work on HK factory guns.

Well I'm balls deep in a CETME L build

After will come the HK stuff if this one turns out ok

There is an old Hk only smith in my area that will not touch clones. I get exactly why, there are a lot  variables I call halfa$$ers between clone manufacturers. There's a flip side to this coin, a challenge that can totally be remeidied. From ptr, century to vector. A full wheel house is a good nitch to carve out. The 91 on an rtg flat is an outstanding rifle, the steel is thicker than ptr, all of the 91 style rifles I've built on an rtg flat shoot tighter than a finished ptr built with the same components (my stock ptrs rebarreled using the same rcm barrels). That is my experience and observations using their flat, makes for a more ridged receiver expecially after welding an optic rail. The rifle is noticeably heavier once completed.

The smaller 9x are easier than the full size imop. The 5 on an lsc is really excellent. Go for it, take your time and I'm sure it'll be nice.

While were here

I'm confused on the whole cocking tube thing

Ive been hard pressed to find a video of a MP5 build, but it seems many cut back flats after they are rolled to weld on a cocking tube.

Do you just cut it to around where the trunion starts and then weld the cocking tube in place?

I have only built on LSC flats and the cutting required is the alignment tabs after you bend them.

Gotcha

Does it "plug in" or is it just straight welded at the seams?

It plugs in, with a circular sleeve inserted into the top part of the receiver about an inch or so. Usually round copper stock is placed inside to act as both an alignment guide and a heat sink during the circumference welding. (A front alignment guide should be utilized as well. One is shown in the photo above.) The position of the cocking tube must be set correctly, relative to the receiver and trunion, so there is a slight gap between the front of the bolt carrier and the cocking lever assembly.

Gotcha

This cetme L build has went pretty well so far. I think I can do a HK build.

Last little question

On this cetme L. I pressed the barrel into the trunion last night. Now the tutorials on youtube seem, odd, in regards to headspacing or making the bolt gap.

They just ram it home and check with a go gauge.

Seems weird to me to use a go gauge so I didn't and just rammed it in the trunion like they did

Ive landed around .010 without a gauge, which is on the lighter end. To get it to say .020 would I just need to push the barrel back out very slightly?

That's one way. I would still check it with NATO go and no-go gauges.
The 2 other primary things which affect bolt gap are the roller size and the shape of the locking piece. You want to be just over minimum NATO headspace with standard rollers or one-size-smaller-than-standard rollers.

Bear in mind that headspace and bolt gap aren't the same thing. You could have a rifle built with perfect everything, then ream a huge deep hole where the chamber used to be, and it would still have perfect bolt gap (but not headspace).
In function, the extractor holds the case to the rear, against the bolt face, but I still like to get the headspace correct.

Understood. Is there any measurements I should be seeing?

With a GO gauge it's over .024.  

Without it's around .010. I'd like to increase this so I guess I'll push it back out very slightly

I do not know common/average measurements for a CETME L. Definitely see if you can find that info, but if you're using a new-production barrel, then its dimensions (if different from factory barrels) come into play as well. Weaponeer, WeaponsGuild, and FALfiles may have info on building CETME Ls. I haven't checked.

You're observing the difference a cartridge will make on the bolt gap using your gauge, so that's good. I would take that GO gauge, add a couple layers of masking tape to the base of it, shave the edges off with a fine file (so you have no interference from the side or edge), then put that on the bolt face with standard rollers, in full lock-up in the trunion. Press the barrel in until it stops against the gauge/rollers/trunion. Then let it sit there without pinning it (hopefully it's very tight). Try a NO-GO gauge without moving the barrel forward. If it won't engage the rollers too much (gap will be way high) then you're good.

Check your gap dimensions against what you find on the net, but if it has some you're OK. The real concern is future wear. (There will be some initial loss due to break-in also. The amount of initial loss depends on whether you start with new/used parts, the hardness of the steel, and how correct the geometry of each part is.)

If it has less gap than you want/need, here are your options:
larger diameter rollers (keep in mind you'll need even larger rollers later as it wears),
beefier locking piece (where it engages the rollers) if available,
or you'll need to remove material from the rear of the breech/barrel.

You need enough gap to account for normal lifetime wear without the bolt ever touching the barrel as the gap shrinks. It is expected you will need to increase roller size at least once in a roller-delayed blowback's service life. This will restore the gap. Too much gap is better than too little, upon initial assembly.

When you're sure it's set correctly, THEN drill & pin the barrel to the trunion. If you don't have a tight enough barrel/trunion interface to check the headspace, then you don't have a tight enough fit to hold the barrel in either, and will need to knurl the barrel to tighten the fit.

Alright so I pushed it out slightly then used a hammer and a bolt until it settled at .018

I think I'm gunna have to call it good enough at that bolt gap

Spec I have seen floating around seems to be round bouts the same as HK roller lockeds

0.018 would be great for an HK build. If you compared with both the GO and NO-GO gauges, then you're probably right where you need to be.

Ill check with the no go when I get home. I guess last night I didn't take the GO gauge out so I had one with me

With a go gauge it seems fine. Again the bolt gap increases more than without though

That wasn't so bad

Just needs some of the welds dressed a bit better and a refinish

Nice! 

Nice L! I put one together and nixed the rear sight for a 15 slot rail, chopped the front sight flush, welded a 3 slot rail and use mbus pro sights along with an acog. I think the L has been the easiest to build so far.

I still need to go back over the welds and try and make them good looking. Everything seems good to go though function wise

Everything movies smooth and freely. Stocks really tight but that'll wear in. Bolt gap checks in at .017

The front sight posts a little wonky. Looks like it got bent and broke the very tip. I'll file on it a little to see if I can get atleast the broken spot rounded out. If it can't zero I guess I'll hunt for another sight post

That's funny you said "wonky" because that is exactly what I called the front and rear sights.. haha

If you were able to chamber bore sight it, you should be very close. Your L sounds like it should be a keeper! The L is such a weird odd ball but at the same time pretty damn cool. To bad they will probably never import them again after the huge Catalonian bust.

Lol well I ended up with 2 cocking tube springs and 2 rear sight screws along with a few other random pins.

I sat there and looked at them thinking WTF!? Where do these go!?

I lucked out on having 2 cocking tube springs as I broke the first one getting the pin in. I kept charging it and it wouldn't ever go completely into battery. Honestly thought I had built it wrong or something. Put the 2nd spring in and everything worked great. Blew my mind that a broken cocking tube spring could be the difference between a function rifle and non functioning.

The sights are.... Yeah. I couldn't believe it was as simple as they are.

Exactly, extra or missing parts for a kit you've never seen before can be confusing, as far as the support spring goes, a 9x spring should work. I have not tried it but it looked basically the same to me. I really wanted to build another but I'm all tooled up without a kit. Are you using a mig or tig? Just go easy on the amps when finishing the rear sight, the top of the receiver will contract and make you bolt group tight. I use a square bar to use a mandrel once all the welding was finished. I typically use 30/35 amps with the tig for everything except the trunnion, and then it's around 75.

I had to sand the top of the receiver to get the stock to slide on butter smooth without wobble. The finally will add just a hair so it should be in the sweet spot when all said and done.

I used MIG on a low setting. Seems to have worked okay. I just need practice making them look decent. I still wanna go over a few and grind them back down to hopefully make it look okay. I know its not the best way to go about it, but its what I have access to.

No no, don't get me wrong. I wasn't nit picking it, looks good man. Who's to say what's best, if you can make a firearm you love regardless what you use, that sounds what'sbest to me. Nothing wrong with a mig, dance with the date ya brought eh!