which 9mm?

[barnbwt]

i wonder, can the KP31s weight be reduced at all? thinner stronger tube or shroud maybe? perhaps a lighter stock? any way to get it down closer to 8lbs?

Yes; the shroud can be removed almost entirely except the stump which locks the sorta-quick change barrel in place. The compensator out front itself weighs a quarter pound and is entirely unnecessary (was added by committee, then removed by soldiers & during subsequent manufacture). The barrel is about 5/8" diameter, kinda fat for a 10" SMG barrel (especially on semi-auto) but allows for multiple drums to be run back to back into advancing Bolsheviks without issue.

The drum magazine is heavy, but trust me it's worth it; basically the best 9mm drums ever designed (which is why the Soviets copied them verbatim for the PPSH, not to mention the entire gun which became the 'PPD'). The stick mags are so good, B&T is still designing new civilian-marketed SMGs which use them. The stock is fairly dense beech, though I suspect much of that is cosmolene; it's a simple reproduction stock to make, though. The receiver is incredibly overbuilt; think RPD. It's on average about 1/4" thick, and the whole thing --tube and the trigger group section below the stock-- was machined out of one giant 30lb forging. The thickness makes the thing heavy of course, but also very tolerant of less than perfect welds --perfect for a first foray into building (I built the Hotchkiss first not the Suomi, since its tube is almost as beefy, but was saw-cut rather than torch cut). If you get one of the 'horizontal' cut kits which has the entire round portion of the receiver gone, you could easily use a new tube of smaller external diameter for the bulk of it (except the end-cap) to keep the weight down.

To be honest, though, if you want to cut the weight that badly (the Suomi was a defensive trench-broom like the Thompson; the weight was to allow for sustained full auto usage) you should go with the later KP44 with the folding stock, or even the Swedish K (of CIA infamy) which use the same mags but are much more lightly built for an 'assault' role. The Swedish K is about one-third the MP5 as far as price, but easily 2/3rds the gun.

and looking at it closer it looks like the finnish just copied the thompson except put it in a round tube, the bolt with its long forward protruding section to eliminate the need to machine clearance for the magazine on the bolt, the section of receiver around the magazine well, the magazine well itself, its all thompson

All tube guns look alike, and all were emulating the MP18 Schmeisser early on (except the Thompson which was and remained its own thing). The two were developed & manufactured completely differently, and have very different recoil, FCG, & cocking handle setups. Thompson was almost all machined initially, the Suomi had a set of giant reamers that bored out all the cylindrical features at once. I believe the Thompson also had a rather funky mag release at first if memory serves, whereas the Suomi's is a more typical paddle. Cocking handle is nonreciprocating & located/shaped like a Mauser handle. The rate reducer was originally pneumatic; the bolt was so closely fitted to the receiver tube that a one-way valve in the rear caused a vacuum to develop on the return stroke. Sadly, not very effective when you gouge a big slot into the bolt for the semi-auto conversion.

BTW, the long 'snout' on all the early subguns was a safety & reliability feature; in the event of case-rupture, breech gases only impinged on a reduced area & did not as greatly exaggerate the force applied to the bolt. It also allows for a much narrower path from mag lips to chamber, with basically no way for the round to jam as it's fed straight in from the mag; incredibly reliable. There are still relief cuts for the mag lips, but not as pronounced as on, say, the STEN or M3A1. I suspect ammo (and chamber) quality improved by the second generation to the point case ruptures & mag issues were no longer so common as to require a feature that stretched the gun two whole inches.

TCB

 

[barnbwt]

so why dont i just choose a bolt from one of these sub-guns, get a tube for it, spot weld on the magazine well of my choice, then fab up a trigger housing or wooden stock to house an MP5 trigger pack?

This approach is basically a standard 'scratch build,' and not hugely different from what building a kit is in the first place, believe it or not. I'd strongly recommend Suomi or PPSH mags, since Jack Squat actually makes weld-up magazine well flats for those two (maybe some others by now, as well) which takes some of the guess-work out of the magwell geometry. PPSH sticks can also feed 9mm without issue (the drums, not so much). One tactic being used more frequently these days is to reshape a standard Suomi/etc bolt (which can be had cheap) to fit a more common tubing size or shape, then fit the built around that. The Suomi is some crazy-high hardness level, though, and will require annealing in the BBQ grill overnight to do any mods to (re-hardening not required)

"my preference would be for a double-feed bolt which would limit me to the sterling, SA24/26, KP44, and maybe the KP31 bolt"
Vintage single-feed mags actually tend to be more reliable, since the lips are much more durable (we're talking stuff the better part of 100 years old, here), but PPS43 mags are a dual-feed contender to consider.

I do recommend you read up on HK trigger packs before going this route for a first scratch-build. There's like a half-dozen ways you can accidentally turn one into something the ATF calls a machinegun while doing the semi-auto conversion, and none of them are intuitively obvious. A pre-made semi-pack is certainly a viable option, though (granted, your trigger will suck & that crazy hammer spring will probably bash your firing pin to smithereens )

just saying that access to a lathe, mill, and welder, a lot can be done with a little ingenuity and some off the shelf components that are readily available

Don't forget two grinders for every welder!

TCB

 

[justin22885]

heres what im thinking.. scratch build, sterling bolt, MP18 inspiration, either MP5 or uzi mags, mlok or keymod slots on the forearm section of the tube, full length top rail, and hack the back end off a polymer garand or M1A stock and hog that out for a polymer MP18 style stock, end result will be a modernized MP18 basically

independent of this idea i do want to pick up a KP31 kit since theyre still cheap, and an SA26 kit or two while theyre still only 100%, KP31 would probably just be built as is, but for one of the SA26 kits i want to see if i could cut it down really small, remove the stock, handguard, and make something the size of a micro-uzi that could be used as a utilitarian sidearm

but yeah, i think im going to go with a "scratch build" patterned after the MP18 with those modern changes

 

[justin22885]

another option i guess i could persue since parts are so cheap is mating a VZ24 upper to an MP5 magazine well and trigger housing plus trigger pack.. a piece of steel tubing, weld on the magazine well and lower section from an MP5 receiver stub, weld on the shelf for an SA trigger frame and trigger pack, and id just have to make up a new collar to slip and pin over the back end to hold it all together

this would be much cheaper than the MP5, simpler in construction and operation not needing a bolt carrier, locking wedge, rollers, or locking recess in the trunnion, but still allow me to use the same mags, stocks, grips, trigger packs, and i can get a set of fixed HK style sights i could mount directly to the top of the tube

 

[usp9]

The simple and reliable Beretta CX4 would do the trick. Prices are good, mags all over the place and takes a host of accessories.

 

[justin22885]

im trying to decide on length of barrel, on one hand id like suppression, 147 grain tuned to about 1100fps, unfortunately this requires serious down-loading to work in an 8-12 inch barrel which would make it incredibly underpowered for pistols, or it would require a very short barrel in the carbine which leads to more noise.. ideally for suppression a longer barrel plus subsonic ammo keeps things even quieter

so what im trying to determine is if i should go with a short 5" barrel in the carbine and a suppressor or essentially an integrally suppressed barrel, or if im better off splitting the load up into 124 grain supersonic for pistol and most carbine uses with special 147 grain loads loaded to be subsonic out of the full length carbine barrel (around 10")

 

[barnbwt]

"so what im trying to determine is if i should go with a short 5" barrel in the carbine and a suppressor or essentially an integrally suppressed barrel"
Why not build a simple 9mm first that doesn't require an additional 400$ in taxes alone & a year wait, before trying to re-invent the MP5SD? The goal is to actually get a gun built, and on a budget...right?

"another option i guess i could persue since parts are so cheap is mating a VZ24 upper to an MP5 magazine well and trigger housing plus trigger pack"
The VZ24 'upper' is basically a pipe; and the only thing left in the kits of use is a partially-torched trunnion (with barrel stump) and the endcap. A much better idea would be to use STEN parts which are much more easily converted to semi-auto, and also still pretty cheap (or to just buy 6" of ~1.5" diameter bar stock & make one). Round tube is definitely the way to go for an easy build, though.

FWIW, MP5 magwells aren't all that cheap, probably running about as much as the kits you are considering. The Jack Squats are like 10$ each. Nor are HK lowers inexpensive, which run the better part of a Benjamin without any of the semi-auto stuff done to them, anymore. You might be able to shave some bucks using a CETME lower, but HKPro will ridicule you & you'll break your thumb trying to use the safety. Barrel is another several dozen dollars, if you are willing to roll with whatever is cheapest from a place like Numrich. You're talking about 300$ just getting started, before conversion parts (scrapyard builds are actually kind of expensive unless you've already accumulated some cast-off parts from other projects)

"new collar to slip and pin over the back end to hold it all together"
Just a word to the wise, pinned caps don't often do so well on these guns; a number of guys have caught a bolt/endcap in the jaw (low velocity, more embarrassing than harmful) when they simply pinned the thing with some drill rod (also a bad idea to use brittle drill rod for cross-pins). I think the HK guns get away with it by having fairly powerful recoil springs to cushion the very light BCG (adds up to a less violent impact being handled by the takedown pin) but a simple blowback will be far more taxing on the endcap/receiver tube. Considering that you'll need a lathe to make the tubes/caps already, it is no trouble to just thread them together & not have to worry.

A really cool build idea using fairly cheap or easy to make stuff would be;
-4130 DOM round tube receiver (1.25-1.5" inside) -30$
-4130 trunnion (welded/threaded up inside the receiver) -20$
-Cold-roll steel stock for bolt -10$
-AR firing pin (10$)
-Uzi barrel/nut (easily made on lathe from a blank) -50$
-Jack Squat Uzi magwell -12$
-An 80%/100% AR lower, ideally w/ damaged magwell (0$-30$)
-AR FCG parts kit -40$
or
-HK/CETME lower assembly (60$-80$)
-A 2ft chunk of 2x6 and some Bondo to form into your stock (probably a bullpup w/ mag through the grip like a CX4 since the AR FCG will be hanging down behind the magwell/grip area

Should be doable to keep it under 200$ if you make as much yourself as you can, and don't count your own labor/tooling . Would also be as accurate as whatever sighting scheme you use, quite reliable, and a good quality trigger pull compared to most SMGs (esp. open bolts or striker conversions)

Suomi can be done in a couple weekends with very little need for tracking down parts or making new ones for about the same cost, though, in case you're actually interested in getting a gun built.

TCB

 

[theotherwaldo]

I went with a KP31 for me, with three sticks and a drum, and an old Hi-point for variety. For more variety I modded an extra Hi-Point stock to take G17 mags. Total price is still under $550, which is less than ammo costs so far.

 

[justin22885]

you know what?.. i think im just going to start a collection getting all of them and making a few of my own.. i was looking for a new type of weapon to get into since im kind of bored of building AK and AK variants, maybe subguns will be my new venue, a KP31, KP44, sterling, VZ26, and a few custom ones

i still want to build a pseudo MP18 with the sterling bolt and modernize it with keymod/mlok slots and a top rail, id also like to make one as absolutely small and compact as possible, like a VZ26 the size of an uzi pistol, but i also wouldnt mind taking the VZ24/26 parts, move the grip back for something the size of an MP5k or tec 9

the simplest one i can think of though is just a 1.5" tube, extended forward for a handguard, extended rearward to mount an endcap embedded inside an MG34 style stock and just put the magazine wherever, on the side, in the grip, or in front of the trigger

 

[barnbwt]

...belt feds are less boring...


TCB

 

[justin22885]

well, maybe this summer ill get the parts kit to an MG34, my favorite belt fed.. sub guns are sort of my primary firearm though, for any situation theres little a pistol caliber cannot handle when fired from a more accurate platform with the energy boost you get from the longer barrel and even though some may not be the lightest things out there you must admit they are easily the simplest both in function and construction

 

[Bohemus]

"Do you mean vz.26 - light machinegun?
I suppose you mean submachine gun vz.48 Sa 23/24/25/26 "
Don't take it personally, but we Americans never did figure out how the Czech naming system worked. And it didn't help that the importers procured stuff with 'VZ" (the year) being the only identifier (and I think a lot of the guns themselves didn't have the model on them, either). So we have made up stuff like CZ52 handguns, and VZ52 rifles. But also UK59 LMGs, and CZ26 in both SMG and LMG form. Then you have ZB37. I have learned that everything but 'VZ' corresponds to weapon type (pistol, rifle, SMG, LMG, HMG) but the marketeers in this country don't seem to consistently refer to their wares by anything but the model year.

TCB

vz.+year is military designation.
Like your M16 or M4
Sa / TK /UK /LK /SHKH / Pi etc.. is "what it is" e.g. Sa - samopal - submachinegun , T - heavy, L - light, U -universal, K - machinegun. Pi - pistole, SHKH selfpropeled wheeled howitzer and so on.
ZB - Zbrojovka Brno , CZ - Česká Zbrojovka, ZV - Zbrojovka Vsetín - producer.

There are three vz.52 - pistol, SMG and LMG.
Sa23-26 vz.48(a/b) is family of SMGs adopted in 1948 and later redesigned from 9mm to 7,62x25mm.

 

[barnbwt]

That system makes far more sense than ours, which increments in order of adoption & gains suffixes as they are revised, but reset when replaced with a new platform. We've had a bunch of M1's; rifles, pistols, MGs, tanks, etc. The numbers also seem to reset randomly, hence M4 coming after M16 (I'm guessing since the short carbine is classified as a 'newer' platform it wasn't adopted as the M17, despite being the same darn gun). I'm not military, so I assume there is a rhyme & reason, but as a poor schmoe trying to make sense of it all it's pretty random

"There are three vz.52 - pistol, SMG and LMG"
Forgot about the LMG; that's the belt-fed BREN/vz.26, right?

"well, maybe this summer ill get the parts kit to an MG34, my favorite belt fed"
Ooh-hoo...$$$$$$ Talkin' two-grand just for a mangled kit. They are cool guns, though, so I can't blame you (I wants a Lewis, meself).

"sub guns are sort of my primary firearm though, for any situation theres little a pistol caliber cannot handle when fired from a more accurate platform with the energy boost you get from the longer barrel and even though some may not be the lightest things out there you must admit they are easily the simplest both in function and construction"
Back-ish on topic(ish). Glad someone else realizes bigger isn't always better in firearms (and in fact usually isn't). You check out the new 9mm belt-fed upper? I was kindly given ten of the little links to experiment with, and I think that ultra-light LMG concept is really cool; depending what kind of capacity you'd want your little carbine to have, a belt can actually be the more reliable/efficient solution.

"i think im just going to start a collection getting all of them"
There's a very sexy ZK383 kit on Gunbroker; very nice guns, almost up there with the MP34 as far as cool factor (MP34 is widely regarded as the most 'stylish' and well-made of all SMGs, but $$$ for kits). Suomis/etc are perfectly fine guns, but I view them as stepping stones to even cooler stuff, which you should stock up on while it's still available, should you plan to do this hobby in the future.

Another cool one, though possibly a bit underpowered for you (9x18) is the PM63 'Rak' which are extremely well made guns brutally destroyed by ATF direction, but if brought back as a semi-auto would make for a very cool MP7-ish gun from the Warsaw Poles. Very well thought out gun, executed excellently, even for the Polish. No one's figured out a good FCG conversion yet, so the kits are still dirt cheap (very recent arrival here in America)

TCB

 

[justin22885]

after much thought, i atleast have some direction as to where im going, since i want a top rail and a bottom magazine this eliminates the option of using sten and sterling bolts, and since a telescoping bolt would offer no advantage here ive decided the KP44 bolt is the one i'll use, its perfectly horizontal charging handle slot bored straight through allows me to put the charger on either side i want and its ejection angle should clear a top rail and optics

still trying to determine if i'll make a pseudo MP41 out of it or make something a bit more compact and MP5 size with the magazine and trigger closer together but i did decide the KP44 stuff is better than the other options as i believe someone had previously mentioned, and at $75 a kit i should buy up a few of them because theres clearly a lot of projects i can do with them