Making 70 to 75 grn 6mm Bullets on .825 Jackets

[jackieschmidt]

We have historically made 66 and 68 grn 6mm bullets on .825 J-4 Jackets.

I have about 10,000 71 grn cores left over from when I was attempting to make a decent 103 grn bullet.

Right now, I am making a 68 grn FB bullet on J-4 .825 jackets, which I have about 16,000 of. I also have access to a double radius BT die set.

I was thinking about cutting some of these cores and squinting them with enough weight to make a 70 to 75 grn bullet. I would have to try a different weight core to see how far the lead comes up into the bullets tip to arrive at a weight. With a 68, there is still quite a bit of air between the top of the core and the metplate when pointed up.

Any of you bullet makers ever did this? Going under the assumption that just about everything imaginable has been tried with the basic 6PPC, I figure somebody has.

I’m sure everybody is thinking…...”Why”?

I have always thought that 133 was a little slow in burn rate for our typical 6PPC. I was thinking that adding a few grns of weight to the same length bullet might be a better match. Everything else would stay the same.

I would probably be looking at around 3370 fps as the upper load window, as opposed to the 3430 I shoot now.

this is how much air gap is in my 68 now.

 

[expiper]

will a 13.5 twist bbl. stabilize a 75 gr bullet ????...Roger

 

[jackieschmidt]

will a 13.5 twist bbl. stabilize a 75 gr bullet ????...Roger

I was going to chamber up a 1-12.

 

[JEFFPPC]

I have a 12 ready to go on my ppc. I would love some 75s. Great Idea!

 

[gunsandgunsmithing]

Berger made 74s for a long time. I think I have a few and will measure one for length. They shot great back then. I was still using mostly 14vtwist bbls then too.

 

[expiper]

im sure they will work in a 12 twist bbl---just wandering if they would work in my existing 13.5 bbl--could you try a few and let us know --any info would be great to know--thanks again --Roger

 

[nmkid]

Odd man out...Would they work in a 1:10?

 

[Jud96]

I think it’s a great idea and I’d be interested to see how they do. I’m new to short range BR and have shot long range for years, not in formal competition just for fun, and to me the idea of using a heavier bullet would help with the wind that plagues everybody. If the 75s shoot as well as a 66-68 and buck the wind better, then I don’t see any disadvantage. Keep us posted!

 

[RGRobinett]

We have historically made 66 and 68 grn 6mm bullets on .825 J-4 Jackets.

I have about 10,000 71 grn cores left over from when I was attempting to make a decent 103 grn bullet.

Right now, I am making a 68 grn FB bullet on J-4 .825 jackets, which I have about 16,000 of. I also have access to a double radius BT die set.

I was thinking about cutting some of these cores and squinting them with enough weight to make a 70 to 75 grn bullet. I would have to try a different weight core to see how far the lead comes up into the bullets tip to arrive at a weight. With a 68, there is still quite a bit of air between the top of the core and the metplate when pointed up.

Any of you bullet makers ever did this? Going under the assumption that just about everything amaginable has Ben tried with the basic 6PPC, I figure somebody has.

I’m sure everybody is thinking…...”Why”?

I have always thought that 133 was a little slow in burn rate for our typical 6PPC. I was thinking that adding a few grns of weight to the same length bullet might be a better match. Everything else would stay the same.

I would probably be looking at around 3370 fps as the upper load window, as opposed to the 3430 I shoot now.

this is how much air gap is in my 68 now.View attachment 1471461

Jackie, several years back, using the .825" J4, I made some 73 Gr. FB for some pals who did very well at both point-blank BR (group) events and at the VH Jamboree. The Niemi point-die is a tangent 8.5 ogive.

To kill two birds with this stone, in answer to expiper's question (#12 above): "will a 13.5 twist bbl. stabilize a 75 gr bullet "????... Roger, Yes, as compared to it's shorter cored/lighter sibling, such a bullet will have slightly GREATER Sg - especially for bullets based upon the same jacket, from the same die, it's always about the bullet length, not the weight! Stability will not be and issue.

Especially via a 1:12" twist, but also via 1:13.5" twist, 72-73ish GR. bullet should shoot GREAT!
RG

 

[ebb]

The top of the lead looks pretty square in the picture of the bullet you cut, are they all that square? I am a carpenter and have been for over 40 years, square and level are real important things to me. Just thinking as a carpenter that has done some shooting. If the lead was not square it would seem to me that the bullet would be out of balance (center of gravity not in center of bullet) at the rpm bullets are pushed today would this be a big accuracy thing? I am not being negative towards your bullet making, I know you to be precise in everything you do. And also know that most all bullets have the air gap to some degree or another. I have just never seen one cut like that and started wondering, and I am sure you know about these things

 

[Tim Singleton]

Jackie
This is sort of an answer
I have tested with a 790 jacket in 2 tenth grain increments with a rail gun
64 to 67
67 had lead in the meplat
66 would shoot but any heavier wouldnt group
66.2 didnt shoot well and lead was not in the meplat area yet
This is a BT so pushed lead line a little higher than a FB will
Short answer is make some up and test them in your rail my guess is anything heavier that a 70 with a BT wont be to benchrest standards.

 

[jackieschmidt]

The top of the lead looks pretty square in the picture of the bullet you cut, are they all that square? I am a carpenter and have been for over 40 years, square and level are real important things to me. Just thinking as a carpenter that has done some shooting. If the lead was not square it would seem to me that the bullet would be out of balance (center of gravity not in center of bullet) at the rpm bullets are pushed today would this be a big accuracy thing? I am not being negative towards your bullet making, I know you to be precise in everything you do. And also know that most all bullets have the air gap to some degree or another. I have just never seen one cut like that and started wondering, and I am sure you know about these things

Of course, when we seat the core in the jacket, it is absolutely square. It moves up toward the tip as the die forms the ogive. That picture shows some burrs from the grinding wheel. It actually appears dead flat.

However, during the point up operation, you are at the mercy of what ever it wants to do. Hopefully, the cores all extrude up in an even fashion.

In short, it is (probably) one of those things you have no control over.

 

[jackieschmidt]

I think it’s a great idea and I’d be interested to see how they do. I’m new to short range BR and have shot long range for years, not in formal competition just for fun, and to me the idea of using a heavier bullet would help with the wind that plagues everybody. If the 75s shoot as well as a 66-68 and buck the wind better, then I don’t see any disadvantage. Keep us posted!

Not really looking for any kind of BC advantage. The whole idea is to add a little weight to the bullet without changing anything else to optimize the load window of N133.

 

[gunsandgunsmithing]

Here's an old box of Berger 74s. I measured 3 and they were.850, .851 and .853.
Berger recommend twist was 1 in 13 but they did shoot even from 1 in 14 twist bbls at my elevation. I will add, it may have been that they were right on the edge because I recall trying them at 600, just for fun and they were hitting all over the place. Everywhere but the target. Lol! Out to 400, it shot well enough that I still remember the load... It was a 6BR with 30.8gr of h322...fwiw.

 

[RGRobinett]

Jackie, I left out the testing part - yes, it concurred with Tim's results (#11 above) there seems to be a maximum fill volume, after which precision suffers - for my die, and .825" J4, that was 73.0 Gr.. That said, those were also 1:14" twist barrels, thus, produced marginal Sg. It's likely that shifting the balance forward exacerbated (muzzle blast/pressure disturbance) and/or, slowed the yaw/pitch damping - much less likey via the 1:12"twist barrel!!
Also, getting the core long enough to extrude near, or, through the me'plat opening , requires a very noticeable increase in pressure - enough so, that i was concerned that the die might rupture!! For thethirty Cals., this was more prominent yet - for the .925: J4, via the Niemi 7 ogive die, the limit was 123 Gr. RG

 

[Jud96]

Not really looking for any kind of BC advantage. The whole idea is to add a little weight to the bullet without changing anything else to optimize the load window of N133.

I think that’s a good idea as well, because N133 is definitely a full case load in most instances with the 66-68gr bullets most commonly used. I think if you could get around 29.0-29.5 with the 73-75gr it would be more optimal than 29.5-30+ that’s used with the 66-68s.

 

[jackieschmidt]

6

Jackie, I left out the testing part - yes, it concurred with Tim's results (#11 above) there seems to be a maximum fill volume, after which precision suffers - for my die, and .825" J4, that was 73.0 Gr.. That said, those were also 1:14" twist barrels, thus, produced marginal Sg. It's likely that shifting the balance forward exacerbated and/or, slowed the yaw/pitch damping - much less likey via the 1:12"twist barrel!!
Also, getting the core long enough to extrude near, or, through the me'plat opening , requires a very noticeable increase in pressure - enough so, that i was concerned that the die might rupture!! For thethirty Cals., this was more prominent yet - for the .925: J$, for The Niemi 7 ogive die, the limit was 123 Gr. RG

Randy, my FB is a copy of the original Fowler and Watson, around a 7 1/2 ogive with a relative straight shank at about .2431 with a .0005 base.

 

[dcali]

I've not done it with those specific bullets, but I have varied the amount of lead in a jacket and tested it against the theoretical calculations.

Long story short, the lighter bullets were more accurate by about the amount that the math predicted - a small amount, but significant by benchrest standards.

Long story long, I wrote some software to try to calculate a measure of accuracy potential for bullet designs based on some work I dug up in an old paper by Bob McCoy. There is a number (which I'll call aerodynamic sensitivity for the purposes of this post) that is analogous to a BC for any bullet design, but for accuracy potential. My aim was to determine the optimal amount of lead to use in a given jacket/die setup that balances accuracy with BC. It turns out there is no "optimal", only tradeoffs.

BC increases linear with weight. adding 5% to bullet weight ads 5% to BC. Very simple.

Aerodynamic sensitivity (which is bad) increases with bullet weight because when you add lead, it necessarily lengthens the weight distribution of the bullet. This is *not* linear, and accelerates the longer you make the core. So as you add lead, the BC steadily increases, but the aerodynamic sensitivity increases faster. So at the upper limits of weight, you gain a lot of accuracy potential by dropping the core weight, and you only pay a little in terms of lost BC. As you continue to lower the weight, the accuracy potential increases, but not by as much. So there's a tradeoff that is more dramatic at the heavier range of the scale, but still present at the lighter end.

All in all, for short range, there's not much point *all else equal* in putting more lead in the nose. For long range, there is, to a point - at some point each pip of BC is costing a lot in terms of the bullet's accuracy - they get finicky quickly.

 

[RGRobinett]

6

Randy, my FB is a copy of the original Fowler and Watson, around a 7 1/2 ogive with a relative straight shank at about .2431 with a .0005 base.

Jackie, - roger that! You'll have a little more volume to play with. I agree with the concept of getting more use from the propellant. RG

 

[dcali]

Here's a chart from an example I ran that's probably a bit more clear. It's interesting that there is a minimum theoretical sensitivity (maximum accuracy potential), but that tends to be below the pragmatic limits of core weight. Note that this presumes an optimal twist. If you limit yourself to available twist rates, there are some jumps in the green curve when you hit a point where there is a better available twist.