Check headspace???

[lysander]

Surprising given the credentials of who I quoted. Correction noted. Why is the M249 chamber shorter?

It is wrong because the person giving the information only gave you half the information. If you convert the length as shown in millimeters to inches, you get the numbers he quoted, he just did not bother to convert the reference diameter.

As to why . . . This article gives some background information and part of the reason, in that a short chamber prevents excessive case stretch. You also have to remember that the clearances around a machine gun's locking lugs are generous, as they have to contend with dirt and muck dragged into the action by the belt.

The other part of the reason is that machine guns tend to have a lot of energy in the bolt, they usually have a 200 round belt lift requirement. Because of the energy requirements, they tend to have high bolt closing velocities. Something has to stop the bolt's forward motion, and that something is going to get beat to **** after repeated impacts. So, they allow the cartridge case to absorb some of the energy when the shoulder gets set back.

[the AR-15 Junkie]

I have sets of Forster gauges, Go, No Go, Field. If you purchase headspace gauges, do not mix brands.

Whats wrong with mixing brands? If they are machine to spec why would it matter?

[AndyLate]

5.56 and any .223 chamber all headspace the same. The difference is in the neck and throat, while headspace is where the shoulder is set.

300 BLK will need to use 300 BLK gauges.

Thanks, Todd

Andy

[T2C]

Whats wrong with mixing brands? If they are machine to spec why would it matter?

I don't mix brands based on a recommendation made to me over 30 years ago from a good gunsmith. It was also mentioned in a Brownell's newsletter I received a few years back. Uniformity of work was mentioned in the article. https://www.brownells.com/aspx/learn...aspx?lid=12555

[lysander]

I don't mix brands based on a recommendation made to me over 30 years ago from a good gunsmith. It was also mentioned in a Brownell's newsletter I received a few years back. Uniformity of work was mentioned in the article. https://www.brownells.com/aspx/learn...aspx?lid=12555

To be fair, the industry has progressed a bit in the last 30 years.

Whats wrong with mixing brands? If they are machine to spec why would it matter?

Because the "specifications" are internal to the gauge manufacturer. If you take a PTG .223 Remington GO gauge over to Forster and have Forster check it for conformity, it may not pass. This is less of a problem since 2015 when SAAMI started to publish recommended headspace gauge drawings with tolerancing, and material properties. (It is still voluntary though.)

This is different from US military gauges, in that it is mandatory that all made to a common drawing, and common standard.

[Disciple]

Because the "specifications" are internal to the gauge manufacturer. If you take a PTG .223 Remington GO gauge over to Forster and have Forster check it for conformity, it may not pass.

Despite the differences if they both confirm a valid chamber what is the problem with mixing? If one of them does not then isn't it a faulty gauge? I don't see how tolerance stacking applies here.

[T2C]

To be fair, the industry has progressed a bit in the last 30 years.


Because the "specifications" are internal to the gauge manufacturer. If you take a PTG .223 Remington GO gauge over to Forster and have Forster check it for conformity, it may not pass. This is less of a problem since 2015 when SAAMI started to publish recommended headspace gauge drawings with tolerancing, and material properties. (It is still voluntary though.)

This is different from US military gauges, in that it is mandatory that all made to a common drawing, and common standard.

Yes sir it has. That's why I posted the link to the Brownell's article, it's a lot more current.

[lysander]

Yes sir it has. That's why I posted the link to the Brownell's article, it's a lot more current.

Not by much, the article was written over twenty years ago. (The .PDF was created on 15 February 2002, and I have a hard copy of it from around 1999.)

Like I noted, IF all the various gauge manufacturers use the SAAMI drawings provided in ANSI SAAMI Z299.4-2015 - "Voluntary Industry Performance Standards for Pressure and Velocity of Centerfire Rifle Ammunition for the Use of Commercial Manufacturers" (published 14 December 2015), you probably could buy from different manufacturers. But, the first word in the title suggest the safe thing is to stick to the same maker.

[lysander]

Despite the differences if they both confirm a valid chamber what is the problem with mixing? If one of them does not then isn't it a faulty gauge? I don't see how tolerance stacking applies here.

Colt's "FIELD" gauge is 1.4736"
SAAMI's "FIELD" gauge is 1.4700"

Apparently, not everyone is on the same bandwagon, at least, not yet.

[T2C]

Lysander,

I ran into a situation where I could not find Go, No-Go and Field gauges for 6.5 Creedmoor all made by the same company, so I decided to wait until I was able to purchase all three gauges made by the same manufacturer. Would you be comfortable mixing Pacific Tool and Gauge with Clymer headspace gauges or Clymer with Forster gauges?