Desired throat ??

[Wayne S]

We all have to deal with the throats in the weapons as they come from the factory. With the most accuracy in mind, IF you could specify a specific throat design what would it be ?
You could design it to have a taper to fit the contour of tapered design like the Eagan series of MX designs, like the Saeco # 315, or a more conventional design that would allow the nose of the bullet to engrave the riflings while the front driving band was a snug fit on the throat ?

[Wayne Smith]

If I were omniscient I would choose one boolit that I knew would work in that barrel and have the throat cut for it. I'm not, so I have to experiment with different designs to see what works in my barrel.

[Outpost75]

Attachment 81665When John Ardito was winning all of the benchrest records in the Cast Bullet Assn., he was using a standard taper pin reamer with 0.007" included taper per inch for throating, and using a bump die which up[set bullets to match the throat angle.

When C.E. Harris designed his 7.62x39 bullets, be similarly tapered the bore-riding forpart to match the angle on the 7.62x39 Lapua pressure/velocity and accuracy test barrel reamer which he bought from JGS, and chambered his 14" twist Hart and 13.7" twist Obermeyer 5R barrels which he cleaned everyone's clock with. The Lapua 7.62x39 reamer has no cylindrical ball seat, but a 0 degree, 45 Minutes BASIC (per side) angle, which has a 0.3114" major diameter at the entrance to the forcing cone. I have had .303 British, 7.62x39, 7.62x54R and .30-'06 reamers ground by JGS with this throat, which shoot everything well from 1934 era Ball M1 to cast bullets from the new HM .312-160-5.

These guys knew what they were doing. Don't fight success!

[45 2.1]

These guys knew what they were doing. Don't fight success!

A very good summation.................. But that has been known for a very long time. Another thing which is little known is that barrel throats "wash" in a predictable shape. You measure enough of them, then plot them to the ten thousandth and that shape becomes known to you.

[Wayne S]

Thank you all, My quest for knowledge on the throating is for a winter project to replace two T/C Contender barrels that have the usual "very long throats" with two custom Contender barrels and have their throats designed for one of two cast bullets, either the NOE 225-55 FN or the NOW MX-3-55. One problem with the 55 FN is that unlike the bullet shown by "outpost" and it's engraving marks from contact with the riflings, the nose on the 55 FN cast from "lino" is to small to contact the riflings. Leaving the tapered MX-3 design.
I might attempt to "beagle" the mold with a strip of ?? just on the bottom of the mold in an attempt to increase the nose Dia.

[MtGun44]

Alright! Asymmetric beagling! A new technology is born.

Bill

[KYCaster]

Alright! Asymmetric beagling! A new technology is born.

Bill

ummmm..........not new. It works very well for me with a 170 gr. bore rider in my 308 win.

Jerry

[Wayne S]

ummmm..........not new. It works very well for me with a 170 gr. bore rider in my 308 win.

Jerry

please explain ??

[runfiverun]

there is also lapping of the mold, or just nose bumping to change the shape/diameter of that portion of the mold.
i lapped the nose section of my rcbs 0-55s.
i then specced the HM-2 mold to duplicate the larger diameter on the rear part of the nose to align the boolit better in the ar type rifles so the nose wasn't a bore rider it was done similarly to the way ED Harris designed the 7.62x39 Lee boolit
it works very well in the 220 swift and 22-250 throats with that rearward nose increase, the body diameter also helps bore alignment.
as long as the alloy is strong enough to withstand the firing forces that type of design works very well especially in the semi-auto's.

[Outpost75]

.................. Another thing which is little known is that barrel throats "wash" in a predictable shape. You measure enough of them, then plot them to the ten thousandth and that shape becomes known to you.

I was told by Horst Zimmerbauer at Lapua years ago that the long, gradual forcing cone developed by the Germans for the "S" bore Mauser 8mm rifle while mostly to control the rate of pressure rise with early smokeless powders, it was also intended to mimmick the natural erosion wear pattern of the forcing cone or leade. Finnish 7.62x54R and 7.62x39 barrels have that 0 degree, 45 minutes Basic profile, in which the major diameter of the forcing cone opening is the same as the maximum bullet diameter. As the bore wears, match shooters increase bullet diameter to maintain the ogive to leade relationship. This is why the D46 match bullets were made in several progressively larger diameters in 0.05mm increments, although only the .308" and .309" diameters were ever exported to the US. The .308s being made only for the American market a nonstandard size, as the British, Canadians, Aussies and Kiwis preferred 7.8mm (.307") and the Europeans 7.85mm (.309")

Pres Kendall did bring in small batches of the other diameters in the 1980s, and I still have a few hundred which I got from him, in the largest sizes 7.9mm (.311") and 7.95mm (.313")

[Wayne S]

I think several my my "situations" may be solved, a shooter will be attending our match tomorrow who has one of the barrel makers barrels like I'm interested in. I'll just take several rounds loaded with my "beagled" 225-55 and MX-3-55 and see ones produce the desired effect.

[shooter93]

If you're having the guns barreled give the smith a couple dummy rounds and have the reamer ground to the fit you want including the case and the throat area.