Please help me understand this Match Chamber

I recently bought a barrel for an H&K G3 type project listed with having a "Match Chamber". When I checked the bullet seating depth to the lands with a 168gr SMK I found that with the bullet touching the lands there wasn't enough left in the case to hold it in place in the cartridge.

In order to see what was going on I cast the match chamber and a standard NATO G3 chamber for comparison.



After taking the diameters I took the casts into work and checked the contours from just behind the shoulder and extended all the way into the land of the rifling to determine where the tallest point of the rifling began.

G3:



Match:



The freebore area of the match chamber is a cone from the end of the neck to the rifling that is longer than anything I've ever seen. Is this something new? I bought the barrel expecting a tight neck diameter and with the expectation of being able to seat the bullet to the lands if need be.

Don't know what is going on there. I think you should talk to the barrel maker.

Depending on the casting material, and the time between taking the mould and measurement, there could be variation.

Have you done your research and qualification?

CalG said:

Depending on the casting material, and the time between taking the mould and measurement, there could be variation.

Have you done your research and qualification?

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Please explain what you mean by research and qualification.

I know Cerrosafe changes some, shrinks at first then grows up to about .002" from what I've read. This is the first time I've needed to use it, so there is a lot to learn. The taper from the neck to the land doesn't seem to indicate much growth because the diameter at the lands is slightly under .308" still. I have read that the growth is a percentage of the thickness of the area, but not sure how to qualify it yet.

I talked to the barrel maker before I made the chamber casts because the groups looked more like 00 buck patterns. I was told the chamber would shoot best with Berger VLD's so I bought some 175gr bullets and a couple boxes of Federal Gold Medal Match with 185gr Bergers installed.

I have tried to make this barrel shoot. I switched scopes today, topping the rifle off with a freshly serviced Leupold 24x. I sighted the rifle in at 50 yards. I'd planned on moving to the 100 yard line but the groups didn't look promising enough to make it worthwhile. Best 50 yard group of the day was the FGGM185's at 2.061"

So it comes down to the barrel or the shooter. Most of the rifles I've built for target shooting, bolt guns and semi auto give me group sizes averaging .8~1" with hand loads. The barrels were all McGowen blanks chambered by myself with a standard Clymer reamer in 7.62x51 NATO or 7.62x54r. I do have one store bought savage model 12FV in .223 that has done some nice groups of .24~.26" range.

I was just curious if anyone had seen a forcing cone type of freebore used before, and what it might be needed for. The barrel maker is dug into his position that there's nothing wrong with the barrel and hasn't said a word since I sent him the pics above.

The barrel maker is dug into his position that there's nothing wrong with the barrel and hasn't said a word since I sent him the pics above.

I think that says about all that needs to be said about this. Do you want to out him?

Just for jollies, you might shoot some groups with Sierra 220 gr MatchKings since they are longer and might reach the rifling while still seating in the neck.

Holescreek said:

Please explain what you mean by research and qualification.

I know Cerrosafe changes some, shrinks at first then grows up to about .002" from what I've read. This is the first time I've needed to use it, so there is a lot to learn. The taper from the neck to the land doesn't seem to indicate much growth because the diameter at the lands is slightly under .308" still. I have read that the growth is a percentage of the thickness of the area, but not sure how to qualify it yet.


I have tried to make this barrel shoot. I switched scopes today, topping the rifle off with a freshly serviced Leupold 24x. I sighted the rifle in

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Your first paragraph of this post addresses the intent of bringing out the research needed.

But it is important to note that dimensional changes with aging of the CerroSafe is not "optional". Changes will occur absolutely in units of inches per inch, Percentage can be used as well.

The home work would be the dimensional change factors as a function of TIME for the EXACT type of CerroSafe you have used.
It's picky perhaps, but the devil is in the details.

Long throats are used to lower ultimate chamber pressures. Weatherby uses them extensively.
Of course, they are not typically seen as "match rifles".

If I was going to buy a chambered barrel to use as is I would get the reamer print for the chamber first before buying. The term 'match chamber' does not in itself mean anything in particular unless you get the particulars.

GGaskill said:

The barrel maker is dug into his position that there's nothing wrong with the barrel and hasn't said a word since I sent him the pics above.

I think that says about all that needs to be said about this. Do you want to out him?

Just for jollies, you might shoot some groups with Sierra 220 gr MatchKings since they are longer and might reach the rifling while still seating in the neck.

Click to expand...

I started taking the rifle apart tonight before I closed up shop. My intent was to remove the barrel and replace it with the used G3 barrel I measured to see how much the groups improved. I hadn't considered the 220's because they aren't something I'd ever shoot normally. I'll need to mull over whether it's worth the effort.

I don't intend to "out" the barrel maker here, I genuinely want professional opinions from experienced gunsmiths that have dealt with chamberings more "exotic" than I ever have. I even removed the name of the company from the contrace report before posting to remove any bias.

sounds like a standard military chamber to me.........but that might be his definition of match....a standard military barrel for rapid fire shooting.