Have been reading about the M231, the open bolt variant of the M4/M16, and notice is a very different animal.

It is open bolt, but not blowback, it has a rotating bolt and appears to have an inertia firing pin just like an M16/M4.

I have often read that one reason you have to have the auto sear on an M16/M4, is that it could fire out of battery if the hammer is released too soon and the bolt is not in battery yet. So why is this not a risk with the M231? Also, it appears not to have a hammer, just a bolt release that is actuated by the trigger group. This is apart from the standard bolt release that is activated by the mag follower.

So if there is not a hammer, how does the firing pin, which appears to be a standard M16/M4 affair, strike the primer with enough force to fire the round?

Anybody well versed in the M321 know how to explain this better?

Look carefully at the photos at https://www.ar15.com/archive/topic.html?b=3&f=123&t=622233.  Compare the third and sixth photos and note the relative position of the "striker" (I don't know the correct nomenclature).

That doesn't answer the OOB question, but makes it easier (for me, anyway) to see how the firing pin is actuated.

Another relevant thread re OOB firing http://www.ar15.com/archive/topic.html?b=1&f=5&t=698045.  The disconnector prevents hammer follow, which can cause the weapon to fire.  No hammer means no disconnector required.  If everything is working properly, the firing pin can't protrude out of the bolt to fire a round unless/until the bolt is locked.

Topic Moved

OK so now it makes sense. The striker is probably designed so it doesn't hit the firing pin until the bolt is in it's fully rotated position, meaning it would have to be in battery. The striker would driven forward by the collection of springs in the short buffer tube. So it is actually a hammerless striker fired rifle.

I wonder why they did not go with a blowback system with a fixed firing pin like the Thompson or M10. Safety maybe? Or just to use existing parts? Either way, fascinating simplicity of design compared to a standard M16.

I was going to say magic. But you had to go and burst my bubble.

For the same reason your AR-15 is not blowback.  The weight required in the bolt to operate .223" as a blowback system would be several pounds.

The bolt would have to be REALLY heavy.  The AFV they developed to complement the MBT-70 used M3 Grease Guns and port firing weapons.  IIRC, the military also considered a roller locked port firing weapon offered by HK.

I've always thought it would be fun (if not practical ) to kluge together a Colt Light Support Weapon with an M231 operating system for the 231s high rate of fire.

About 5.6lbs for the bolt itself, using a standard rifle buffer and rifle buffer spring, I read.

My 9mm bolt is 14oz, compared to 11.5oz for a standard AR BCG.  A 5.6# blowback bolt would need to be made of, or have a weight added, of something pretty dense and heavy.

5.6# of reciprocating mass.  No thanks.

OK so blowback is only feasible for pistol calibers unless you have roller locking or some such.

@ which point you have roller-delayed blowback.  Even the little Savage .17" HMR semi-auto isn't straight blowback - it's delayed blowback.  The Remington 597 .17" HMR semi-auto was straight blowback, and was recalled b/c it tended to extract too early & blow the magazine out the bottom.

The largest caliber suitable for straight blowback that I've field stripped is - 40mm.  The Mk19 40mm belt-fed grenade launcher is straight blowback, w/ a bolt on the order of 15 lbs or so.  But it's intended to be vehicle or tripod mounted, and the pressure of the grenade shell is relatively low, so it works.

I think the firing pin is pushed via the action springs when it slams home..

The striker cannot touch the firing pin until the bolt is in battery.  But the striker strikes the firing pin the moment the bolt is in battery, with no delay, which is responsible for the extremely high cyclic rate.