Oddball - Mac10 Carbine Open Bolt

[Skans]

Here are some pictures of an oddball gun that I acquired. It's an original RPB Mac10 Open Bolt Carbine. Any information that anyone can provide would be helpful. I've never seen another one like this....but I did once see a wooden forend advertised as a part. (in the process of uploading pictures...)

 

[Alleykat]

Stocks have been available for MAC 10s forever.

 

[Sturmgewehre]

I don't see any pictures.

 

[buzz_knox]

I thought ATF had classified all the open bolt MACs as SMGs, on the theory that they could be readily converted to FA.

 

[Skans]

sorry about that - I just uploaded the pictures.

 

[Willie Lowman]

@ Buxx Knox... It is my understanding that the ATF required that no more of those guns were made like that as they were to easy to butcher into a bullet hose.

The ones made before that ruling were grandfathered.

It is still legal to make a open bolt semi auto... It just has to be a good enough design that it can't be converted to FA by the ATF's team of "inspectors"

 

[Skans]

Open Bolt guns are legal, but after 1982, ATF prohibited the future production of open bolt guns which could be easily converted. Willie is correct, if someone could design an open bolt gun that couldn't be easily converted, it's not automatically illegal. It's just that there is no reason to expend such resources trying to figure out how to construct such a gun.

I sort of like the old open bolt guns, although there really weren't that many different varieties made. This RPB Mac10 caught my eye because it was an odd looking carbine and was an open bolt. I have never seen another before - I just couldn't pass it up.

You can tell it was manufactured as a carbine because of the "c" after sa just before the seriel number. Interestingly the little "s" (probably for semi-auto) just before M10 seems to have been an after-thought.

Just wondering if anyone has seen this particular configuration before?

 

[p99guy]

They were a regular item when RPB was producing Ingram's, and could be obtained as a kit to put on your RPB "SAP" (semi auto pistol) with an identical
upper unit and a bolt on butstock. There were 9mm ones as well as the .45 carbines. But that was in the period of roughly 1979-1985. I ended up with a couple of the SAPS back in the day, but never owned the carbine.

 

[ranger dave]

open

i saw a open bolt mac go for $1000 at a gunshow you have a real nice gun the way it is you could get 1300 easy

 

[David the Gnome]

So if I understand this correctly; the bolt stays open after each shot and is dropped forward when the trigger is pulled, is that correct?

 

[ranger dave]

open

yes that is true like a lot of sub guns uzi, grease gun, mp 12 thompson,

 

[psycho nut]

Oddly enough, I like it.

 

[Crosshair]

Holy cow, how much did that run you? I've always wanted an open bolt semi-auto just because of the scarcity issue.

The only open bolt designs that I have seen since the "ban" are single shot guns without any provisions for a magazine. I've thought about designing an open bolt single shot 22 RF fr the heck of it, but have never gotten around to it.

 

[Skans]

Holy cow, how much did that run you? I've always wanted an open bolt semi-auto just because of the scarcity issue.

I honestly don't remember. I purchased it about 5 years ago and I recall thinking it was a good deal, so it was probably around $500. For an old open bolt gun, it's in good condition - many of the old openbolt M10's show quite a bit of ware. I just had never seen one of these old RPB's in a carbine version - the wood forend (with checkering!) is what did it for me - had to have it.

I have one other open bolt gun - Demro Tac1 - it's an odd duck too. I thought about starting a collection of open bolt guns at one time, but prices got too high and there really aren't that many different styles of open bolt guns made.

 

[carguychris]

So if I understand this correctly; the bolt stays open after each shot and is dropped forward when the trigger is pulled, is that correct?

Yes.

Open bolt blowback firing sequence:

1) pull trigger, 2) bolt goes forward under spring pressure, 3) bolt strips fresh round from magazine, 4) bolt pushes round into battery, 5) firing pin is released, 6) BANG, 7) recoil pushes bolt back into rearward position and extracts empty case.

Closed bolt blowback firing sequence:

1) pull trigger, 2) firing pin is released, 3) BANG, 4) recoil pushes bolt back into rearward position and extracts empty case, 5) bolt goes forward under spring pressure, 6) bolt strips fresh round from magazine, 7) bolt pushes round into battery.

The advantage of an open bolt is that a fresh round only contacts the bolt for a fraction of a second, so the primer won't "cook off" and spontaeously fire if the bolt is extremely hot. This is good when a gun has a very high rate of fire.

The disadvantage, from an NFA and/or ATF perspective, is that it's often relatively easy to convert an open-bolt firearm to full-auto by removing whatever trigger mechanism restrains the bolt to the rear between Step #7 and Step #2. It's inherently more difficult to convert a closed-bolt gun to full-auto.

To put it another way, in an open-bolt gun, the trigger restrains the bolt; in a closed-bolt gun, the trigger restrains the firing pin.

 

[Skans]

Open bolt blowback firing sequence:

1) pull trigger, 2) bolt goes forward under spring pressure, 3) bolt strips fresh round from magazine, 4) bolt pushes round into battery, 5) firing pin is released, 6) BANG, 7) recoil pushes bolt back into rearward position and extracts empty case.

Pretty close, but here's a few things you should know: In most open bolt guns, like the UZI or Mac10 (M10) the firing pin is fixed, i.e. it permenantly protrudes from the bolt face. It also has a lip on the lower portion of the bolt that pushes the round into battery, and just before the round is seated, the bolt-face with protruding firing pin shoves it the rest of the way and fires it simultaneously. So, there is no release of the firing pin, as it's really part of the bolt face.

M10 closed bolt opperation actually involves an internal hammer being released and striking the firing pin contained in the bolt. However, in a closed bolt UZI, it's as you describe.

 

[carguychris]

Pretty close, but here's a few things you should know: In most open bolt guns, like the UZI or Mac10 (M10) the firing pin is fixed, i.e. it permenantly protrudes from the bolt face... So, there is no release of the firing pin...

M10 closed bolt opperation actually involves an internal hammer being released and striking the firing pin contained in the bolt. However, in a closed bolt UZI, it's as you describe.

I was trying to make my description as generic as possible; I now realize I should have written "firing pin strikes primer" rather than "firing pin is released".

This would cover bolt-mounted fixed firing pins and the various methods of releasing a non-fixed firing pin (pin on hammer, hammer strikes floating pin, striker-fired, etc).

 

[GONIF]

let me also add, that open bolt weapons are never as accurate as a closed bolt weapon in my experience. all Ingrahm mack 10's that were made in Powder Springs GA were open bolt and a better quality (worth more) than later productions made elsewhere. as is also true of UZI , model A UZI's that were made by IMI were all open bolt weapons and Worth more than later UZI's.

 

[Skans]

model A UZI's that were made by IMI were all open bolt weapons

I have a Model A UZI but it's a closed bolt and all of the ones I've seen are closed bolt guns.

Were any open bolt semi-auto UZI's imported?

On another note, do y'all think that my M10 Carbine could pass for a KRISS???

 

[Moloch]

let me also add, that open bolt weapons are never as accurate as a closed bolt weapon in my experience.

Thats possible, when the bolt accelerates to push a fresh round into the chamber its force can move the gun a tiny bit, if the bolt is very heavy it can affect the weapons accuracy.

The germans invented the FG42, a paratrooper bullpup rifle (the first bullpup) which could shoot semi auto and full auto, in full auto ''machinegun mode'' it shot with an open bolt, in rifle mode it shot with a closed bolt. The closed bolt gave more accuracy and the open bolt helped cooling the barrel. (That was the theory)