Questions about military 300WM chambers.

[Bob2650]

Questions about military 300 Winchester Magnum chambers. Both vintage and current.
Recently came across a box of 400 pieces of FC 08 300WM brass I bought at least 10 years ago. After sizing with a Redding body die it will not chamber. I will need to buy a Larry Willis collet die to use this brass in a normal 300WM chamber. Out of curiosity I measured a few pieces of this brass and found that they must have been fired in a very loose chamber with most of the body some 3.5 thousandths larger in diameter than the SAAMI max allowable spec. No indication of over-pressure was found with the primer pocket, base or belt diameter.
Just above the belt the brass measures 0.518". Allowing .001" for spring back the chamber probably is something like 0.519" in diameter. SAAMI spec for that spot is 0.5136" with a tolerance of +.002" and I found the same amount of oversize at .2" below the shoulder. The base of the shoulder was in spec.

So here are the questions:
Are overly sloppy chambers like that the field fix for over-pressure rounds in extreme heat? This ammo was loaded in 2008 so they were MK248 mod 0 with the 190 SMK .
I have never heard of such a reamer so I suspect someone was using a tool that could have been anything from a stick with sandpaper wrapped around it to a flex-hone from Brush Research. This explains the fire once and toss policy.

Bonus questions: If there was such a reamer, what is it's name or nomenclature and where can I get one?

Last questions: Do some of the MK13 rifles and other vintage rifles have such a loose chamber? Exactly which reamer was used to build the MK13? Perhaps the standard A191 is not the correct reamer for the MK13 after all? Is the military version of the reamer for the MK248 mod 1 also extra loose like this?

Probably too many questions,
thanks in advance,
Bob

 

[Random Guy]

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[nn8734]

Questions about military 300 Winchester Magnum chambers. Both vintage and current.
Recently came across a box of 400 pieces of FC 08 300WM brass I bought at least 10 years ago. After sizing with a Redding body die it will not chamber. I will need to buy a Larry Willis collet die to use this brass in a normal 300WM chamber. Out of curiosity I measured a few pieces of this brass and found that they must have been fired in a very loose chamber with most of the body some 3.5 thousandths larger in diameter than the SAAMI max allowable spec. No indication of over-pressure was found with the primer pocket, base or belt diameter.
Just above the belt the brass measures 0.518". Allowing .001" for spring back the chamber probably is something like 0.519" in diameter. SAAMI spec for that spot is 0.5136" with a tolerance of +.002" and I found the same amount of oversize at .2" below the shoulder. The base of the shoulder was in spec.

So here are the questions:
Are overly sloppy chambers like that the field fix for over-pressure rounds in extreme heat? This ammo was loaded in 2008 so they were MK248 mod 0 with the 190 SMK .
I have never heard of such a reamer so I suspect someone was using a tool that could have been anything from a stick with sandpaper wrapped around it to a flex-hone from Brush Research. This explains the fire once and toss policy.

Bonus questions: If there was such a reamer, what is it's name or nomenclature and where can I get one?

Last questions: Do some of the MK13 rifles and other vintage rifles have such a loose chamber? Exactly which reamer was used to build the MK13? Perhaps the standard A191 is not the correct reamer for the MK13 after all? Is the military version of the reamer for the MK248 mod 1 also extra loose like this?

Probably too many questions,
thanks in advance,
Bob

Contact LRI. Chad has a mk248 mod 1 reamer, which was used for all mod5 and mod 7s. The A191 was used in all previous iterations of the mk13 family.

I can tell you that my mod 5 launched 220 smks at 2970 on average w/my mk248 mod1 clone load when i had it. Very accurate, no pressure signs.

 

[Bob2650]

I have that same print. All the information I have found is that this is the exact A191 chamber reamer that was used on the military sniper weapons. The Z dimension .5141 in the top right corner would have to be something like .519 in the chambers these 400 rounds were fired in.
That is why I was hoping an armorer or gunsmith might throw some information on how those chambers got big enough to leave brass like that.

 

[Bob2650]

As a civilian there are plenty of larger cartridges and I do not feel the need to mess with my 300WM chambers. They burn barrels fast enough as it is. The energy level is close enough to the max my usual range allows for the steel targets. I would love to see the print for a mk248 mod 1 reamer. I believe they have longer throats, not wider than SAAMI in the body. Fat 300WM brass is nothing new and many people have bought once fired mil surplus brass with the issue. I think the oversize chambers are not rare on military sniper rifles.

 

[Bob2650]

I have discovered that one of my old Hornady 300 Winchester Magnum full length sizing dies sizes close enough to all the way down to the belt that I can chamber that brass in all three of my 300WM rifles now.
That die set was put away in storage while I searched for the "perfect" die set. I ended up with Redding as I shifted to searching for a "good enough" die set but the Redding does not size all the way down to the belt, close but not close enough.
Problem solved, this batch of 400 pieces of the military version of Federal Gold Medal brass that is super consistent in weight and neck thickness has been saved from the scrap bucket without what looks like the incredibly tedious use of that collet die.
I will be using this die for the first resizing only then switching back to the Redding as the old Hornady die works the necks hard enough I will want to anneal every time I use it. I put a 7mm button expander/decapper stem in the Hornady 300 WM sizer die to fix dents in the necks and will be using the usual mandrel to expand the necks.
I will quit right there because this is a collectable rifle chamber thread and we have a different place for reloading/brass prep threads.

 

[sirhrmechanic]

Tag for tomorrow…

 

[Bob2650]

I found that a .416 Remington Magnum sizing die is a very good body die for the 300 Winchester Magnum. Does not come close to the shoulder.

 

[Frank Green]

It can depend on when the gun was built and which chamber spec was used…..

I know for fact that at least 3 different reamers have been used…

Saami min spec

The A191 reamer

And Saami spec. +.002” size on diameters! Or what I call a production chamber spec.

Ask me how I know? We’ve made bolt gun barrels and ammunition test barrels in all 3! I was shocked when I was told we needed to make some barrels in the +.002” production chamber spec. That and the Saami spec are horrible when it comes to accuracy. The gun might shoot great…. And it might shoot poorly. It’s the luck of the draw.

As a guess it sounds like the OP has the chamber that is +.002” bigger on diameters if your sizing die won’t size it down properly. Or your sizing die isn’t made correctly or the die can’t adjust down far enough. If you can’t adjust it down far enough take .010” off the shell holder so you have some more adjustment. Wouldn’t be the first time I’ve seen this.

I’m going to beat a dead horse to death here. Don’t kid yourself the Mk248 ammo with the 220gr bullet is loaded beyond max working pressure. The shit is hot regardless if you think you don’t see any pressure signs. 2900+fps the load is frickin hot! On average it runs about 70-72k psi range. If you gave that ammo to me I wouldn’t run it thru my gun. I’d pull the bullet, dump the powder and redo it.

Later, Frank
Bartlein Barrels

 

[Plinker 73]

FYI: the brass from my MK 13 MOD 7 will almost fall into a Wilson case gauge after firing. ‘Tis is with Mk 248 MOD 2 ammo. I have no issues resizeing this brass. However i also have encountered some of the surplus oversized brass.

 

[Bob2650]

The brass I have must have been fired in chambers at least 0.005" over the saami print diameter. That is why I asked about fat chambers and any modifications that may have been made to some of the rifles. No one here has even heard about those chambers?

 

[918v]

Sako TRG have fat chambers like that.

 

[Frank Green]

The brass I have must have been fired in chambers at least 0.005" over the saami print diameter. That is why I asked about fat chambers and any modifications that may have been made to some of the rifles. No one here has even heard about those chambers?

Never heard of .005”. That seems to excessive…. But like I said, +.002”….yes and I’m pretty sure I have the chamber print on my computer at work.

 

[sirhrmechanic]

The brass I have must have been fired in chambers at least 0.005" over the saami print diameter. That is why I asked about fat chambers and any modifications that may have been made to some of the rifles. No one here has even heard about those chambers?

Could the gun they were fired in have excessive headspace? If the cartridge slides back just a tiny amount… the taper of the case body will appear to have massively excessive “diameter” on it.

What’s the background on the box you bought? Surplus? Range pickup? Any idea where they came from?

An out of spec, worn out m24 or experimental xm2010??? (Did I remember that number right) that spit out oversize brass might have nothing to do with a big chamber. Just incorrect headspace or worn out parts. Maybe a poorly-done barrel change?

Add to that the 248 ammo is hot and you can get brass stretched considerably beyond elastic limit with only a tiny headspace issue

Just some hypotheses…

Sirhr

 

[supercorndogs]

Or what about a semiautomatic 300wm. Could it be puffing the case up some as it's extracting it?

 

[sirhrmechanic]

Or what about a semiautomatic 300wm. Could it be puffing the case up some as it's extracting it?

Those old Browning BAR sporters were available in .300 IIRC and those things destroyed brass!

Good possibility.

Then again, most of them never got fired 400 times in 40 years… much less enough to accumulate four hundred rounds since 2008 or later.

Sirhr

 

[supercorndogs]

Nemo makes a semi auto ar10 type 300wm as well, specifically chambered to shoot mk248 ammo from what I have heard.

Mk248 brass is normally junk due to it being loaded over pressure.

 

[Bob2650]

The brass is mil surplus bought from Grafs or Bartlett. It is not over length.
Federal 08.

 

[Frank Green]

I just went and looked at the mil spec drawing for the M2010. Yes that 300wm chamber has a standard Saami throat but the case dimensions from the neck diameter back to the belt area is +.002" bigger. Right before the belt it is .5136" vs Saami at .514" but the A191 chamber is .5117".

Also right before the shoulder area of the chamber it is .4925" vs Saami at .4900". Case neck shows .3421" vs Saami at .34196" right at the bottom of the taper. Case mouth is the same on both at .3407".

Also and I had to look at it twice but the distance from the bolt face to the main shoulder diameter of the body of the case (right before the case shoulder area) is also +.024" longer as well as it's that much longer to the case mouth dimension. I had to look at that again and why it's longer I don't know but it's longer.

Why the variances in the dimensions... I don't have a clue how and why it was all spec'd the way it was.

 

[sirhrmechanic]

I just went and looked at the mil spec drawing for the M2010. Yes that 300wm chamber has a standard Saami throat but the case dimensions from the neck diameter back to the belt area is +.002" bigger. Right before the belt it is .5136" vs Saami at .514" but the A191 chamber is .5117".

Also right before the shoulder area of the chamber it is .4925" vs Saami at .4900". Case neck shows .3421" vs Saami at .34196" right at the bottom of the taper. Case mouth is the same on both at .3407".

Also and I had to look at it twice but the distance from the bolt face to the main shoulder diameter of the body of the case (right before the case shoulder area) is also +.024" longer as well as it's that much longer to the case mouth dimension. I had to look at that again and why it's longer I don't know but it's longer.

Why the variances in the dimensions... I don't have a clue how and why it was all spec'd the way it was.

Spec’d that way “probably” for sand (dust, really) and to ensure reliability.

Minute of ISIS was good enough when you got a quick follow-up or two. And they weren’t shooting 1/8 moa groups for record where those guns were going. A pie plate at 1000 would ensure a hit. And most shots were a lot closer.

Plus IIRC the M2010 was done with the idea that in emergencies it could be a “medium MG” albeit only semi-auto. It could lay down a bunch of rounds fast if a team or a squad needed it.

So big chamber, reliable extraction on dusty shells and desert conditions….

Loosen it up and accept a small loss of accuracy for a big increase in firepower over a .308 bolt gun or even a .308 gas gun.

And your numbers probably explain the O/S cases as well as anything!

Me… I’d lube them well. Put them in my big Herters press With the modified 36” arm and press back into shape. Then load with sane amounts of powder from there out. Neck size or shoulder bump and FL size about every third firing. They will last a long time. If they are 248 cases they are rugged as hell.

Honestly, I’d like some myself!

Cheers, Sirhr

 

[Bob2650]

Would they have used 08 brass for the 220 grain round?

 

[Frank Green]

Would they have used 08 brass for the 220 grain round?

Maybe....

I know it was first talked about (going to the 220gr bullet) back around 2005.

I know the very first test barrels we made for FC was late 2007 or Jan 2008 time frame but that doesn't mean they didn't get some others prior to that from someone else. If they did my guess would be Remington. That's a guess but it would be a good one.

I know it was adopted in 2009 ammo wise and guns etc... where entering service by around 2010.

So I'll say it is totally possible ammo started being made in 2008.

Later, Frank
Bartlein Barrels

 

[Frank Green]

The brass is mil surplus bought from Grafs or Bartlett. It is not over length.
Federal 08.

Can you post a pic of the headstamp on the case please?

 

[Frank Green]

I've got sample rounds for bullet seating depth at home from FC. I'll take a look at those headstamps and see what I got. It was from a while back.

 

[Bob2650]

 

[Frank Green]

The sample rounds I have at home are dated 09. I’ve got a call in to see when it started but I think it was 2008. We we’re well into making the test barrels and they had questions and we we’re going back and forth. That’s when they sent me the sample rounds so I had some for reference. The test bbl orders where really steady from like I said late 07 and early 08 and we ran them for a few years on and off when we got orders until it came to a stop.