Modifying SAAMI Reamers

[Marlin Junky]

What if anything can be done to the following reamer to create a good cast boolit chamber? How about shortening the ball-seat to create a 1-2 degree throat angle?

Thanks,
MJ

[Tom Myers]

What if anything can be done to the following reamer to create a good cast boolit chamber? How about shortening the ball-seat to create a 1-2 degree throat angle?

Thanks,
MJ

By simply grinding back the reamer's 3 degree leade angle cutting edge to a 1 1/2 degree cutting edge, the reamer would cut a chamber that reduces the freebore length of 0.1032" to a length of 0.0411" this is the chamber it would cut. In my estimation, an excellent cast bullet chamber.

When viewing the chamber diagram, note that one must subtract the step length and the small freebore leade where it steps down from 0.2435" to 0.2430" at the front of the cylindrical free bore from the the displayed freebore value to arrive at the actual cylindrical freebore value of 0.0411" .

Just for kicks, I entered the values into my software and came up with a bullet design that fits the re-designed chamber nicely.

http://www.tmtpages.com/LinkSkyImages/forum_images/6mm_Remington/TMT_C244-237-100-RN.Jpg

http://www.tmtpages.com/LinkSkyImages/forum_images/6mm_Remington/6mm_Rem.~_SAAMI~TMT_C244-237-100-RN.Jpg

http://www.tmtpages.com/LinkSkyImages/forum_images/6mm_Remington/TMT_C244-237-100-RN_100_gr_Sketch.Jpg

[Marlin Junky]

So this is not only a feasible but practical modification that any competent gunsmith should be more than happy to perform?

BTW, would that boolit stabilize in a 1:12" twist or would the substitution of a .1" meplat be more compatible? Actually, the heaviest boolit I'm planning to use is the RCBS 243-095.

MJ

[Tom Myers]

So this is not only a feasible but practical modification that any competent gunsmith should be more than happy to perform?

BTW, would that boolit stabilize in a 1:12" twist or would the substitution of a .1" meplat be more compatible? Actually, the heaviest boolit I'm planning to use is the RCBS 243-095.

MJ

It is feasible but probably not very practical.
I would imagine that very few gunsmiths have the ability to accurately re-grind an existing reamer. You would most likely need to send it back to the reamer mfg or else have one custom made.
The barrel would need to be set back and then re-reamed as the modification can only remove metal, not put it back.

The SAAMI specs twist rate for the 6mm Remington is 1 in 9". Theoretically, that twist will adequately stabilize a bullet that is 0.9" in length at a minimum muzzle velocity of 1,536 fps.

If your twist rate is indeed 1 in 12", the theoretical minimum stabilization velocity calculates out to 2,731 fps. for a bullet 0.9" long. So it might be difficult to achieve accuracy with the RCBS 243-095 bullet.

The maximum calculated bullet length for a 1 in 12" twist and a minimum muzzle velocity of 1,536 fps is 0.670 inches.

Below are some sketches of the RCBS 0.90"bullet and a designed 0.67" (0.058" with gas check) bullet in the unmodified 6mm Remington SAAMI chamber.

Hope this helps.

Tom

R.C.B.S.

http://www.tmtpages.com/LinkSkyImages/forum_images/6mm_Remington/6mm_Rem_~SAAMI~R.C.B.S._Mold_243-95-SP.Jpg


http://www.tmtpages.com/LinkSkyImages/forum_images/6mm_Remington/R.C.B.S._Mold_243-95-SP__95_gr_Sketch.Jpg


TMT Design
http://www.tmtpages.com/LinkSkyImages/forum_images/6mm_Remington/6mm_Rem_~SAAMI~TMT_C244-237-75-FN_Throat.Jpg
http://www.tmtpages.com/LinkSkyImages/forum_images/6mm_Remington/TMT_C244-237-75-FN__75_gr_Sketch.Jpg

[Marlin Junky]

Tom,

So it becomes practical if the reamer is modified by the reamer supplier?

This project is starting with a #4 contoured barrel blank and the gunsmith needs to order the reamer.

Thank you for the super drawings. What exactly is the best way to approach this because I don't want to seat RCBS 243-095 to 3.15"; i.e., I want to cover both lube grooves while seating the leading band snugly into the Leade.

Perhaps, I'm barking up the wrong tree because this was supposed to be an M77 short action project (COL: 2.9").

MJ

[shooter93]

I assume you already have the reamer MJ. The manufacturer will regrind it and are reasonable in their pricing. I use a 1-1/2 degree a lot for rifles that I want to shoot both jacketed and cast. Seems to work ok for jacketed too. I have a couple in the works that will be primarily cast guns so I'm going to go to one degree to try. I have reamers made generally ( cost to me is about 100 bucks) which is a small portion of the rifles I have built so I don't mind the cost. I also like them made for case length I pick after sizing and trimming the brass. Most chambers are too long in the neck area to suit me. that does mean you have to be careful about case length but I don't care about that either. I make a dummy round or two and send those to the reamer maker. I have had reamers modified by the maker though.

[Tom Myers]

Tom,

So it becomes practical if the reamer is modified by the reamer supplier?

This project is starting with a #4 contoured barrel blank and the gunsmith needs to order the reamer.

Thank you for the super drawings. What exactly is the best way to approach this because I don't want to seat RCBS 243-095 to 3.15"; i.e., I want to cover both lube grooves while seating the leading band snugly into the Leade.

Perhaps, I'm barking up the wrong tree because this was supposed to be an M77 short action project (COL: 2.9").

MJ

If the SAAMI Spec reamer is modified to cut a 1 3/4 degree leade, Then your R.C.B.S. 242-95 can be seated into a SAAMI spec case to a COAL of 2.8" with the lube grooves covered and the front band will just touch the beginning of the leade cone at the front of the freebore.

The only concern I would have would be the twist of the barrel blank. Is it a 9 or is it a 12 twist? A 9 twist should stabilize the 0.91 inch bullet but a 12 twist may be hard to stabilize unless jacketed velocities are used. I have no experience loading or shooting 6mm bullets so I'm can't say that it will not stabilize but ....

Shooter93 has offered some great advice concerning the reamer. Maybe someone with experience loading 6mm bullets could chime in and share some first hand knowledge of the twist requirements.

http://www.tmtpages.com/LinkSkyImages/forum_images/6mm_Remington/6mm_Rem~_SAAMI~2~R.C.B.S._Mold_243-95-SP.Jpg

[Tom Myers]

Tom,

So it becomes practical if the reamer is modified by the reamer supplier?

I want to cover both lube grooves while seating the leading band snugly into the Leade.


MJ

Just a note to explain why my drawings have a lube groove exposed at the end of the cartridge mouth. If a chamber has a relatively sharp angle (50 degrees or less) step down to the freebore or groove diameter. I have found that placing a lube groove at the step will eliminate a lot of leading. When the fire is lit behind the bullet and bare lead is sitting in that gap between the case mouth and the end of the chamber, sometimes lead will be extruded out into that gap and then be shaved off by the start of the freebore or groove diameter. If lube is present under the gap, it seems to stop any lead rings that might have been formed if lead were present instead of lube.
Not saying that it is a sure fire cure for leading but it seems to work well for my applications. However, most of my shooting is from a bench or cross sticks and I don't have to worry about the lube picking up dirt or debris. If I were shooting in a field situation I would want the lube grooves covered.

[45 2.1]

What if anything can be done to the following reamer to create a good cast boolit chamber? How about shortening the ball-seat to create a 1-2 degree throat angle? Thanks, MJ

Sure........ besides reducing the throat angle, measure your loaded cartridges neck diameter.... add 0.001" to that and have the reamer neck area altered to that diameter. Cut the OAL to maximum trim to length. Case head measurements could be like the neck diameter also. Many SAAMI chambers are overly large and long.... the 7mm Mauser is a prime example.

[Marlin Junky]

Thank you all for the very specific advice.

Just for the record, I'm not going with a 1:9 twist. The barrel will probably be a Douglas 1:10" (six grooves) from Brownells.

Thanks again,
MJ