MAX acceptable Pressure Ring on FB 66g 6mm (PPC)

[Carson]

You can check it with an indicator rod and a bushing set or a set of deltronic pins

I can't picture an indicator rod and bushing set, set up to check the bore. Do you mean like a dial indicator set up on a surface plate and measuring from top to bottom out at the muzzle. I can see checking with Deltronic pins but that would just give me the diameter inside the lands.

Given that land heights vary, I think that my friend's method is the best. He casts laps to measure groove diameters. Of course barrels with odd numbers of grooves present a special problem, in that they require special tooling to to measure their upset slugs or laps. With a cast lap, one can feel how consistent a barrel is, where there are tight and loose spots, and the relative sizes of different parts of a barrel. It is really quite a useful tool, that my friend has learned to use with great skill. He checks out every new barrel before doing any cutting, except perhaps to remove a burr or upset at the ends of the bore. He also uses them to look for problem areas in finished barrels.

After I posted I started remembering something about people driving a lead slug through a barrel. That is out of my league. I think I would just end up with an unusable barrel and an unusable slug. Even if the slug came out perfect I would be sort of hard pressed to get a usable reading off of the core with all of the lands, grooves and gouges I picture. I measure stuff like that pretty much every day but not with the lands.

We have some rubber molding compound we us for odd shapes. It is to soft to put a micrometer on it but it might measure pretty close on an optical comparator.


I thought I was over the ground bullet thing. Now I'm thinking that grinding may still be good if I would just put back on a die ring at the base. The sad thing is that all this takes a lot of work. If even at best you could improve a benchrest bullet 7% it might go unnoticed with my testing. I figure maybe 93% of the spread of my groups comes from not being experienced with mirage and wind. I'm still thinking really hard about it. Very little coming natural. I don't really have much feel for just knowing which way to lean. I'm often taken way off guard by changes everyone else just seems to take in stride.

Anyway you should have seen those ground beauties. I still have a few I could someday but the die ring back on. How big or how tough really is a good question. I suppose the diameter is limited by how thick our case neck is and what our neck chambering is.

While I was flipping through The Accurate Rifle by Warren Page for the place that mentioned grinding or turning the bullet shank. I didn't see it but I did come across this;

page 106

a misshappen point created little or no error a miscut or slanted butt brought about major dispersion.

 

[dixieppc]

Very interesting posts. Even though it doesn't have anything to do with the topic of the original post I have a question out of curiosity about bore diameters. My latest gun build is a medium distance (600 yard) 6BR. When the project was started it was decided to build the gun around a couple of specific bullets, the Berger 105 Match Hybrids or 105 Match VLDs and/or the 108's. The barrel would be a Bartlein but there was going to be a choice of a bore diameter of either .236 or .237. Now, I made a lot of the decisions concerning the building of this gun but the bore diameter I left up to my gunsmith because I did not have a clue. I'm coming into this mid range game from being a point blank PPC shooter so everything is a new experience. He said that if I was going to shoot the burger 105's or 108's then the larger .237 bore diameter would be best.

Bartlein only offers a bore diameter choice in 4 of the 16 calipers it produces of which 6mm is one where you have the bore diameter choice. So.... Why the choice. I can only assume that it is because some 6mm BT bullets are fatter or thinner than others like we see in the pressure ring on FB bullets.

Thanks in advance for enlightening me.

 

[Dusty Stevens]

It has to do with the pressure you desire to push with. Everybody likes different ones- some like more pressure and some like to get velocity. Its just a personal preference

 

[dixieppc]

It has to do with the pressure you desire to push with. Everybody likes different ones- some like more pressure and some like to get velocity. Its just a personal preference

Okay Dusty, I can see that. So why would one choose high pressure/lower velocity over low pressure/higher velocity? It can't have anything to do with tuning because you don't know where a barrel will like to shoot until you screw it on and start testing.

One last question. Why would you suppose that bartlein would offer bore diameter choices only for 4 of the 16 calibers they offer instead of all of them?

Please bear with me. Even though I've been shooting point blank benchrest for some 25 years, I still learn because of the stupid questions I ask. And now that I am entering mid range (600 yards) benchrest, I'm full of stupid questions.

Regards......

 

[Boyd Allen]

There was an unpublished test with a top shooter, a couple of fast twist 6BR barrels, .237 and .236, Bergers and Lapua Scenars were used...in a good shooting rail gun. The first barrel tried was the .237, and the Bergers shot very well, the Lapuas not so much, to the extent that they were contacted and sent more bullets. Then the switch was made to the .236 and the Scenars magically came alive, including the ones that were suspected of being defective. Perhaps that is the answer to your question. Different bullets have different requirements. BTW the Scenars were very uniform in every measured dimension.

 

[jim1K]

There was an unpublished test with a top shooter, a couple of fast twist 6BR barrels, .237 and .236, Bergers and Lapua Scenars were used...in a good shooting rail gun. The first barrel tried was the .237, and the Bergers shot very well, the Lapuas not so much, to the extent that they were contacted and sent more bullets. Then the switch was made to the .236 and the Scenars magically came alive, including the ones that were suspected of being defective. Perhaps that is the answer to your question. Different bullets have different requirements. BTW the Scenars were very uniform in every measured dimension.

Boyd, I'm glad you told me that about the Lapua Scenars, I just got some of the Lapua Scenars L's in 6mm 105 and they aren't even round. As far as .236 verses .237 it doesn't mean a rats rear end but groove diameter does. i slug everything ………. jim O'Hara

 

[Dusty Stevens]

Okay Dusty, I can see that. So why would one choose high pressure/lower velocity over low pressure/higher velocity? It can't have anything to do with tuning because you don't know where a barrel will like to shoot until you screw it on and start testing.

One last question. Why would you suppose that bartlein would offer bore diameter choices only for 4 of the 16 calibers they offer instead of all of them?

Please bear with me. Even though I've been shooting point blank benchrest for some 25 years, I still learn because of the stupid questions I ask. And now that I am entering mid range (600 yards) benchrest, I'm full of stupid questions.

Regards......

Because those are the 4 calibers people have wanted to experiment with over the years

 

[mwezell]

Can someone calculate how much a barrel grows under pressure of firing?
This subject has always been hard for me to get my head around....The bore grows when fired, and the bullet obturates to fit, so how does any of this stuff matter? Serious question. Logically , to me, it seems a bullet with a diameter of same or just under freebore diameter would be best, but it's truly hard to understand how it matters much once the barrel and bullet both begin to change dimension, themselves. I would think consistency from end to end would be the most critical aspect of bullet to bore relationship...much more so than a tenth or so of bullet diameter, before it's pressured upon. If anything, I would think a slight taper to the bore toward the muzzle would be best due to lower muzzle pressures, or perhaps a contour that follows the pressure curve. Just thinking out loud, here. Anyone have any thoughts as to this.

 

[Boyd Allen]

I think that a lot of this is determined by "cut and try" rather than calculation.

 

[mks]

Can someone calculate how much a barrel grows under pressure of firing?
This subject has always been hard for me to get my head around....The bore grows when fired, and the bullet obturates to fit, so how does any of this stuff matter? Serious question. Logically , to me, it seems a bullet with a diameter of same or just under freebore diameter would be best, but it's truly hard to understand how it matters much once the barrel and bullet both begin to change dimension, themselves. I would think consistency from end to end would be the most critical aspect of bullet to bore relationship...much more so than a tenth or so of bullet diameter, before it's pressured upon. If anything, I would think a slight taper to the bore toward the muzzle would be best due to lower muzzle pressures, or perhaps a contour that follows the pressure curve. Just thinking out loud, here. Anyone have any thoughts as to this.

Mike,
A quick rough estimate for 60ksi in a 1"OD barrel with 0.308" bore: 0.00074"

This site will do the stress calculation for you: http://www.engineeringtoolbox.com/stress-thick-walled-tube-d_949.html

Keith