M249 vs M16 Headspace

ericoak · Jan 24, 2018

So a go gauge for an M249 is longer than field gauges normally used for AR15s. Is there a difference in bolt and chamber that allows for the longer head spacing, a function of the gun being open bolt (doubt it), or is an ar15 field gauge conservative for safety and/or accuracy?

http://www.ar15barrels.com/data/headspace.pdf

clandestine · Jan 24, 2018

Lugs on a AR are not as strong as a 249.

Short headspace is more common than excessive headspace with Civilian AR's.

If you don't buy the ColtII Field Spec, that's fine. It's your rifle.

ericoak · Jan 24, 2018

clandestine said:
Lugs on a AR are not as strong as a 249.

Short headspace is more common than excessive headspace with Civilian AR's.

If you don't buy the ColtII Field Spec, that's fine. It's your rifle.

So it is definitely option 1, the gun is built differently. I'm relatively naive on the subject. I thought excessive head space was a problem not because of the bolt, but because the case could either expand and snap the head off making it hard to extract the case, or the case could be unsupported in the rear and crack venting gases into non pressure containing areas of the gun. Sounds like I'm mistaken? I know just enough about this to be dangerous.

clandestine · Jan 24, 2018

ericoak said:
So it is definitely option 1, the gun is built differently. I'm relatively naive on the subject. I thought excessive head space was a problem not because of the bolt, but because the case could either expand and snap the head off making it hard to extract the case, or the case could be unsupported in the rear and crack venting gases into non pressure containing areas of the gun. Sounds like I'm mistaken? I know just enough about this to be dangerous.

You are not wrong about the brass. The excess headspace can allow premature lug failure, as can overgassing on shorter gas systems like the MK18. It may not be related, but I personally feel that High Pressure Testing leads to premature Bolt Failure, but that is mandated for the current M4A1, regardless of the detriment.

The D.I. System helps to reduce bolt lug load compared to other designs. When the Gas System is pressurized, it in a overly simplified explanation pushes the load off the lugs (pushes the Bolt forward) which allows it to cam and unlock (Carrier moves rearward).

HK has tried to increase bot life in their Piston M4 Replacements and didnt gain any substancial life expectancy. IMO the bolt failures on the 416/M27 are due to the gas piston design, which wont unoad the lugs like the D.I. system can.

If one were to use the HK steels on the current M4A1 and skip the HPT I am certain bolt life could be doubled in most applications.

Im rambling. My apologies.

ericoak · Jan 25, 2018

Thanks for the help!

StantonCree · Jan 25, 2018

clandestine said:
You are not wrong about the brass. The excess headspace can allow premature lug failure, as can overgassing on shorter gas systems like the MK18. It may not be related, but I personally feel that High Pressure Testing leads to premature Bolt Failure, but that is mandated for the current M4A1, regardless of the detriment.

The D.I. System helps to reduce bolt lug load compared to other designs. When the Gas System is pressurized, it in a overly simplified explanation pushes the load off the lugs (pushes the Bolt forward) which allows it to cam and unlock (Carrier moves rearward).

HK has tried to increase bot life in their Piston M4 Replacements and didnt gain any substancial life expectancy. IMO the bolt failures on the 416/M27 are due to the gas piston design, which wont unoad the lugs like the D.I. system can.

If one were to use the HK steels on the current M4A1 and skip the HPT I am certain bolt life could be doubled in most applications.

Im rambling. My apologies.

I don't know what you said but my pants are tighter

outrider58 · Jan 25, 2018

Rambling not, I say.

Major03 · Jan 25, 2018

clandestine said:
You are not wrong about the brass. The excess headspace can allow premature lug failure, as can overgassing on shorter gas systems like the MK18. It may not be related, but I personally feel that High Pressure Testing leads to premature Bolt Failure, but that is mandated for the current M4A1, regardless of the detriment.

The D.I. System helps to reduce bolt lug load compared to other designs. When the Gas System is pressurized, it in a overly simplified explanation pushes the load off the lugs (pushes the Bolt forward) which allows it to cam and unlock (Carrier moves rearward).

HK has tried to increase bot life in their Piston M4 Replacements and didnt gain any substancial life expectancy. IMO the bolt failures on the 416/M27 are due to the gas piston design, which wont unoad the lugs like the D.I. system can.

If one were to use the HK steels on the current M4A1 and skip the HPT I am certain bolt life could be doubled in most applications.

Im rambling. My apologies.

Interesting...had to read this twice to get the gist, but I think I got it. So why the high level interest in a gas piston system other than it runs cleaner...which if you lube properly makes zero difference?

clandestine · Jan 25, 2018

Major03 said:
Interesting...had to read this twice to get the gist, but I think I got it. So why the high level interest in a gas piston system other than it runs cleaner...which if you lube properly makes zero difference?

The simple answer is people are misinformed.

There is a much more complex answer when it comes to the military wanting to replace a weapon.

Doco Overboard · Jan 25, 2018

clandestine said:
Lugs on a AR are not as strong as a 249.

Short headspace is more common than excessive headspace with Civilian AR's.

If you don't buy the ColtII Field Spec, that's fine. It's your rifle.

If short HS is more common in civilian AR's will the bolt still generally close enough to allow it into battery or does it sort of resize the case due to the way the mechanism operates? because of the action spring and the mass of the bcg, or does it come into play because of the myriad of manufactures different components tolerance stacking, different build components and cartridge HS through the different ammo makers. I always look at head space as a range not a measurement.

I guess what Im asking is, when I hear someone say my rifle doesn't feed or like this ammo too well and you see them working the charging handle like a well pump and bumping the stock to a fixed surface could a short chamber getting dirty be the culprit? Most semi-auto's should at least have a little radial play for reliability as far as I know.
I would think the bolt lug arrangement is pretty strong just because it has multiple lugs and often wondered how many could be broke and still operate safely but I guess any thing can break catastrophically.
223/556 makes alot of pressure in there, they're no joke.

hit3961 · Jan 25, 2018

clandestine said:
The simple answer is people are misinformed.

There is a much more complex answer when it comes to the military wanting to replace a weapon.

M249 gauge is longer because you don't allow the bolt to lock when checking it..
I can send you the TM if you wanna read it

It comes down to this if Breech Bore Erosion is a no go change barrel.
If head space is no go Change bolt.

clandestine · Jan 25, 2018

hit3961 said:
M249 gauge is longer because you don't allow the bolt to lock when checking it..
I can send you the TM if you wanna read it

I appreciate the offer hit3961. I do have the TM.

Serious question here. Would you agree that the 249 bolt is tougher than the M4/M4A1? How many 249 Bolt failures did you see?

I dont have 1/100000th the experience with the 249 as I do with the AR so I always appreciate the education.

hit3961 · Jan 25, 2018

clandestine said:
I appreciate the offer hit3961. I do have the TM.

Serious question here. Would you agree that the 249 bolt is tougher than the M4/M4A1? How many 249 Bolt failures did you see?

I dont have 1/100000th the experience with the 249 as I do with the AR so I always appreciate the education.[/QUOTE.
Hell yes stronger !!!!

No bolt failures at all just stuck rounds in barrel and feed problems from old magazines and stupid privates forgetting to put feed paw spring back in.

clandestine · Jan 25, 2018

Doco Overboard said:
If short HS is more common in civilian AR's will the bolt still generally close enough to allow it into battery or does it sort of resize the case due to the way the mechanism operates? because of the action spring and the mass of the bcg, or does it come into play because of the myriad of manufactures different components tolerance stacking, different build components and cartridge HS through the different ammo makers. I always look at head space as a range not a measurement.

I guess what Im asking is, when I hear someone say my rifle doesn't feed or like this ammo too well and you see them working the charging handle like a well pump and bumping the stock to a fixed surface could a short chamber getting dirty be the culprit? Most semi-auto's should at least have a little radial play for reliability as far as I know.
I would think the bolt lug arrangement is pretty strong just because it has multiple lugs and often wondered how many could be broke and still operate safely but I guess any thing can break catastrophically.
223/556 makes alot of pressure in there, they're no joke.

Yes, you are right in some instances an AR can smash a live round into a chamber that has short headspace.

Short headspace can cause a few things on the AR. If it does not prevent the bolt from going into full battery, it will usually result in pressure indicators on the brass. This can lead to erratic extraction or faulire to extract due to increased chamber pressure. Some guns exhibit poor accuracy.

In a perfect storm an AR with short headspace can chamber a live round and not be fully into battery. If the firing pin has excessive protrusion (excess of .038) it can allow the gun to fire out of battery. Aftermarket firing pins for guns like 7.62x39 AR's are indistinguishable from 5.56 firing pins unless you gauge them.

In many cases, when an AR is hard to extract a live round, it may short headspace. This is not something a Military Armorer would see as they have stringent QC for components. Tolerance Stacking is not a problem for the Military as it is in the civilian world with budget parts and junk coatings.

The AR bolt is a good design but it does not have a long service life on some applications. When they fail,, they fail arounf the Cam Pin, or the opposing Lugs next to the Extractor. The gun will still run with a few broken bolt lugs as long as they dont end up in front of a chambered round and touched off.

clandestine · Jan 25, 2018

hit3961 said:
No bolt failures at all just stuck rounds in barrel and feed problems from old magazines and stupid privates forgetting to put feed paw spring back in.

Thanks for the feedback. I sincerely apprecate the info.

Cant do anything about the privates messing them up.

hit3961 · Jan 25, 2018

clandestine said:
Thanks for the feedback. I sincerely apprecate the info.

Cant do anything about the privates messing them up.

Like my drill sergeant said " You want your m16 nice and wet like a p$$$y"

Pinecone · Jan 26, 2018

clandestine said:
Yes, you are right in some instances an AR can smash a live round into a chamber that has short headspace.

Short headspace can cause a few things on the AR. If it does not prevent the bolt from going into full battery, it will usually result in pressure indicators on the brass. This can lead to erratic extraction or faulire to extract due to increased chamber pressure. Some guns exhibit poor accuracy.

In a perfect storm an AR with short headspace can chamber a live round and not be fully into battery. If the firing pin has excessive protrusion (excess of .038) it can allow the gun to fire out of battery. Aftermarket firing pins for guns like 7.62x39 AR's are indistinguishable from 5.56 firing pins unless you gauge them.

In many cases, when an AR is hard to extract a live round, it may short headspace. This is not something a Military Armorer would see as they have stringent QC for components. Tolerance Stacking is not a problem for the Military as it is in the civilian world with budget parts and junk coatings.

The AR bolt is a good design but it does not have a long service life on some applications. When they fail,, they fail arounf the Cam Pin, or the opposing Lugs next to the Extractor. The gun will still run with a few broken bolt lugs as long as they dont end up in front of a chambered round and touched off.

My Colt 6920 has headspace on the short end.

If the rounds will not fully go into the case gauge, they may not chamber. And then be hard to extract.

And when I did a carbine course, their dummy rounds (for malfunction drills) would fully chamber, but would NOT extract at all.

I probably ought to bring it by and have you check to make sure it is not TOO short.

clandestine · Jan 26, 2018

Pinecone said:
My Colt 6920 has headspace on the short end.

If the rounds will not fully go into the case gauge, they may not chamber. And then be hard to extract.

And when I did a carbine course, their dummy rounds (for malfunction drills) would fully chamber, but would NOT extract at all.

I probably ought to bring it by and have you check to make sure it is not TOO short.

It's a simple fix assuming the headspace dimensions are the issue. Sometimes a short throat can cause the sticking.

Sometimes the chamber has a more restrictive diameter and that is a little more involved.

I'll gladly help however I can.

If it's a headspace problem the gauges will
detect it. The resolution is a different bolt head or we lap your existing bolt head in.

If the throat is too short, I have a throat reamer to cut it to proper 5.56 specs.

3paul10 · Jan 26, 2018

mpdc4511 said:
I don't know what you said but my pants are tighter

I apologize fellas but I too am aroused.

clandestine · Jan 26, 2018

3paul10 said:
I apologize fellas but I too am aroused.

M249 vs M16 Headspace

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