Lengthening throat in front of chamber

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Wm Cook posted this 5 days ago

I'm hesitant to ask this because the subject is out of my wheel house, but here goes. 

I'm trying to understand what could/should be done to the throating depth in my stock barrel to optimized accuracy.  This is on a Savage 10 F in .308 and it appears as with most stock barrels when it comes to cast bullets, that it has a short throat.  And as I understand it, Production class allows for "lengthening" of the throat.  Thus my need to understand where the best seating depth should be in relation to where the bullet base goes to.

The bullet's a 311299 is 200 grain chambered in .308 and the picture shows the location of the bullet in the case in its push back or "jam" position.  More tension on the case mouth and another .010 seating depth longer and the bullet would stick in the lands.  As its seated now I have nice lands about 2x the width of the land. But as you see the base of the bullet is well inside the case. 

I don't have an issue with base below the case neck but I believe that there must be a rule of thumb somewhere in cast bullet accuracy that someone more knowledgeable than I would look at the depth of the bullet base in the case as it's seated and could comment on whether the stock throating supports or does not support accuracy.   

I suspect that the bullet is too far inside the case and that the answer is to lengthen the throating.  Therefore the COAL could be increased and the bullet base pulled nearer to the base of the case neck.

If I am right, should I have the throat cut to be able to get the COAL longer?  And if I have the throat cut longer would I be ok having the gas check come up to just below the neck.  Thanks, Bill.

 

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John Alexander posted this 5 days ago

The picture looks like the front band isn't even out of the case. Do chambered dummy result in the the front band being touched?  If not the nose must be doing the pushing back. If so -- since, unless your are beagling, the nose can't be made smaller, try softer alloy for the lands can engrave the nose more to allow seating out longer. 

I don't think there is anything but unsupported opinions that there is anything wrong with having the base hanging out in the powder space, and the theories cited to support the opinions are bogus, so I wouldn't worry about that but having more of the bullet supported in the throat/bore and less of it in the neck might be a good thing.

John

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Spindrift posted this 5 days ago

Just my 0,02$ As John says, it seems to be the size of the nose that limits your COL. Resistance to initial engraving (nose/ lands) should be minimal. I wouldn't worry about it. Have you consulted Prof. Target at Range University?

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Wm Cook posted this 5 days ago

With the current COAL the driving band is inside the case and the lands are pushing the bullet back to its jam or push back position.  I have to get some casting done this morning but by the end of the day I want to try a tighter grip on the bullet and re-verify where that push back or jam dimension is. 

I'm not so much concerned with the base sitting back in the case as I am in seeing how deep the bullet is in the case and saying to myself "hey, for optimum accuracy this just isn't right".  Then I start extrapolating from my base knowledge and can't help but think that if I had 10+ years of chasing cast accuracy I would see what I'm looking at and know what to do.  Example: 1) lengthening the throat if indeed a 16 bhn alloy still pushes the bullet back into the case as far as it is or 2) If it shoots leave it alone. 

I apologize for the dumb questions because I am on the short end of the learning curve for cast accuracy.  And chasing accuracy is the only reason I shoot.  Not too much on how many pounds of alloy I leave in the back stop but getting all five bullets to go through the same hole.  Nothing wrong with either objective.  Everyone has their own itch to scratch.

And I still owe everyone closure on that lead buildup on the 6 O'clock position where the case cut for the chamber ends.  That problem is still in process.  Thanks, Bill.

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Spindrift posted this 4 days ago

Will, I think you posted an excellent and relevant question. I’m not an expert in cast bullet accuracy either, I just think the bullet is worthy of a chance without making irreversible changes to the rifle. 

Best of luck!

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Ed Harris posted this 4 days ago

I would first drive a bullet up into the throat to the point where it is forced into the rifling, knock it out and measure it.  I expect also that the diameter of the forepart on the bullet is too large. A bullet having a smaller diameter forepart may help.  So may sizing the nose to enter more gently.  If neither of those approaches work, reaming to only reduce the angle of the origin of rifling and to lap any tool marks would be my next step.

73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia

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Shopdog posted this 4 days ago

There's a few things you can try. As mentioned above,the nose may be cau

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Shopdog posted this 4 days ago

Oops,

nose may be causing issues if it's too large. But that's easily corrected with a tapered nose die. This tapering can,if necessary include part of the top drive band.

Try,seating a bullet backwards in the case. First with the sized GC,if that doesn't show you something,try it without the GC.

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Squid Boy posted this 4 days ago

You might want to try pushing the bullet into the muzzle end. Whole lot easier to see where and how hard it is hitting. It looks like a bore riding design so I would think there should be little resistance to entering the muzzle. Just an idea, Squid Boy

"Squid Pro Quo"

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Wm Cook posted this 4 days ago

I still need some advice and until I get another new barrel to work with I can only assume that I understand things a little better now.  What's that line from Bob Lee Swagger; "About the time you think you have it figured out, you don't".

Facts;

  • With FL sized brass the COAL was 2.705.  Greater than that the bullet would stick and I would use my cleaning rod from the muzzle end to tap the bullet nose while manipulating the bolt to wiggle the bullet free of the lands.  Seating depth was poor with the driving band inside the case neck and the bullet base into the case body.  Land tracks were clearly evident at about 1 x 3.  No contact with the driving band because it was inside the case.
  • After 15 or so rounds it felt as if it was not at a sticking "jam" but I still had the same 1 x 3 tracks so I felt good to go.
  • After ~60 rounds and after spinning chore boy while attempting to clear a lead buildup at the 6 O'clock position where the case neck cut meets the lede I still see 1 x 3 land tracks.  But I can now advance the bullet reaching a COAL of 2.840 without sticking the bullet.  The bullet is tight into the lands but the bullet does not stick and it does not push back into the case.  Land tracks are now 1 x .180" or maybe 1 x 5's.  The first bullet lube groove is just inside the case neck and the gas check is starting into the case neck. Everything looks good to go.

Assumptions

  • The 60 rounds fired plus the chore boy brushing polished the lede, lands to make it act like it had more rounds through the barrel than the 60 that was fired.
  • Cast bullets are softer than jacketed and should permit more advancement into the lands as long as the initial burrs from the chambering is smoothed out, so with a polished bore the cast bullet should slide into the lands further without sticking.
  • I should be good to go if other cast shooters can confirm that they too are able to seat the bullet far enough forward that the land track length that I am seeing (.180" without sticking) is not abnormal.

If it's abnormal, well that's another deal completely.  I am going to be starting up another new .308 with cast by the end of the month and I'll keep better tabs on the jam / push back allowances as the barrel gains mileage.  Thanks, Bill.

 

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Ross Smith posted this 4 days ago

I'm with squid boy if the case mouth tension is firm enough you should be able to seat the bullet out further. I also found it advantageous to seat the first band out far enough to seal the gap between the case mouth and the start of the bore(leade?). Have you tried or considered a shorter bullet?

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