this is an article from Mike Bellm I found interesting. A while back I mentioned I loaded 115gr .312 (32-20) bullets in my Ruger Single Six in 32 H&R with excellent accuracy. This raised some eyebrows and started a minor ruckus about T/C and Ruger using a .308 bore, this should put those concerns to rest. The most interesting part to me was the experiment Mikes mentor PO Ackley tried.
Main topic.......Groove diameters..........
and this applies to all guns, not just the 7.62x54R Russian.
First off, the Ruskies make their .30 cal. bullets usually .310" diameter, which includes most of the 7.62x39 AK type ammo as well. Many of the barrels for the 7.62x54R Russian reportedly have groove diameters upwards of .312" or larger, Russian bullet diameters also are reported to sometimes be upwards of .312, yet I am advocating rechambering .308" groove diameter US barrels to the Rusky.
This raises alarm among some fearing excess pressures as a result of putting a larger diameter bullet down a smaller groove diameter barrel.
The Finns it is reported rebarrelled a lot of the Russin Mosin Nagant rifles with .308" groove diameter barrels, named them 7.62x53R, and conceded the 54 ammo could be shot in their rebarrelled rifles with the smaller groove diameter. That is my understanding at this point anyway.
Ruger, I have mentioned before, made the Mini-14 in 7.62x39 (.310" bullets) with .308" groove diameter barrels.
Bullets larger in diameter than the barrel's groove diameter is not a problem at all, IF the throat is not too small for the bullet.
Thus, being the cheat that I am, when I cut chambers for rounds that might use different bullet diameters, I cut two and sometimes 3 different diameters in the same throat.
For the Ruskies, I go in full travel at a scant .308" diameter, go in about half way with a .311" diameter, then lastly open up the throat to .312" for at least a minimum length to accommodate the larger bullets.
I do the same thing with .357 Max. throats knowing guys will be shooting both .357" and .358" bullets.
Few other than the Bellm Bunch are aware that TC's .35 cal. barrels are undersize. Specs are supposed to be .358" groove diameter, .350" bore diameter, yet a .348" reamer pilot will not go into a TC .35 cal. bore, and the barrels usually slug out at about .356" to .357" at most with some less than .356".
This includes the .35 Rem. Contender barrels shooting .358" bullets in a gun that is highly pressure sensitive, yet no problem at all.
From time to time I repeat the story of an experiment Ackley did eons ago. He took a .30 cal. barrel, copper crusher measured the pressure firing a 180 gr. .308" bullet through it from .30/06. He then opened the neck and throat in the barrel, shot an 8mm (.323") 180 gr. bullet through it with NO increase in pressure measured. The experiment to prove the point about bullet diameter v. groove diameter ended with opening the neck and throat for .35 cal. and shooting a 180 gr. .358" bullet through the .050" smaller bore size and registered a LOWER pressure .... for reasons he never could establish. All diameters of 180 gr. bullets were fired with the same powder charge.
Oversize loaded neck diameter and cases too long and crimped by the end of the chamber neck are the things that create excess pressure. With no room for the neck to expand and let the bullet start moving, pressures go up dramatically.
As I write this, and having stated the throat must be large enough in diameter, one thing does not compute, such as the .45/70 chambers with NO throat at all. Likewise, vintages of .30/30 chambers in TC barrels had NO throat whatsoever..... only an overly long chamber neck.
So with further examination, it can probably be proven that only constrictions in the neck create excess pressure.
One prime example of this is the age old problem of guys necking down .308 Win. brass to make .243 Win. cases and ending up with a loaded neck diameter too large for the barrel's chamber neck. I've run into this myself, and it does jack up pressures dramatically.
Then comes the question about the short .223 Rem. commercial chambers v. the longer throated 5.56 mil-spec chambers where the mil-spec ammo runs excess pressure in the short throat. Extend the commercial throat at least .100", and the commercial chambers shoot mil-spec 5.56 ammo just fine with no pressure issues at all. In this application, throat LENGTH is the issue.
Back to the 7.62x54R Russian ammo fired in .308" groove diameter barrels. Have no concerns. The chambers I cut take the larger diameter bullets into account.
Further to that, there are no doubt a few of you wanting to shoot .303 British and 7.65 Argentine Mauser in the Encore. This is the same situation with bullet diameters running the same as the 7.62x54R, and I happen to have reamers for both as well as .30 cal. unchambered barrels from Bergara that I can custom chamber for these rounds as well. If you want to shoot 7.65 Argentine or .303 British, I can help.
All the best,
Mike Bellm
