Question for boolit designers

[Marlin Junky]

I was wondering if there are any guidelines with respect to base length for plain base cavities; and, with respect to gascheck shank length, is 1/8" better than 1/10"? I'm looking for maximum performance from 44 and 45 cal. rifle boolits in the 300 to 400 grain range... I guess that means about 1-1.5 MOA at 1600 to 1700 fps (roughly). I recall from reading V. Smiths book that he likes long gascheck shanks for maximum performance but I don't recall him publishing specifics. I will be probably be using SR4759 for the PB boolits unless I discover a better powder somewhere down the road.

Thanks,
MJ

[leftiye]

There is a design for the gas check shank that includes a small groove just in front of the check. This is to allow more **** (fouling) to be scraped off of the barrel before running out of place to store it and carry it out of the barrel. In any event, it is probly a bad idea to not have sufficient space in front of the gas check to accomplish this task. I'd look closely at Veral's designs and maybe copy them. He probly thought about this before he finalized his designs. Look also at Mountain Molds' gas check design with the groove.

[Marlin Junky]

There is a design for the gas check shank that includes a small groove just in front of the check. This is to allow more **** (fouling) to be scraped off of the barrel before running out of place to store it and carry it out of the barrel. In any event, it is probly a bad idea to not have sufficient space in front of the gas check to accomplish this task.

Right, but if a groove can't be created in front of the seated check because of machining limitations, are there any down sides to creating a straight shank that's somewhat over 1/10"? Say, 1/8"?

Thanks for the response... I still need PB input.

MJ

[sagacious]

Right, but if a groove can't be created in front of the seated check because of machining limitations, are there any down sides to creating a straight shank that's somewhat over 1/10"? Say, 1/8"?

Thanks for the response... I still need PB input.

MJ
MJ,
If I'm reading your response right, you may have read Leftiye wrong.

If the gascheck shank is long, say 1/8"-- and thereby longer than the check is deep-- then that extra shank length forms a 'lube groove' itself. Some of the LEE 44cal molds have this feature. A Hornady check will not totally cover the entire shank length, and thus the extra shank length forms an extra lube groove. Since the LEE 310RF shoots like a house-afire, and when loaded correctly doesn't lead my guns, one is forced to conclude that this 'feature' is not in any specific way deleterious to internal or external ballistic performance.

Whether that groove immediately in front of the check functions to remove and accumulate fouling is open to conjecture, but I think that argument applied generally might prove a hard row to hoe. I have seen many times where a good cast load that uses no gas check will remove any previous lead fouling upon firing. A gas check with a groove in front of it is certainly not required for that.

The difference between 1/10" and 1/8" is only twenty-five thousandth's, and in this case that twenty-five thousandth's is not part of the bearing surface. That dimensional disparity on the gas check shank length is not likely to make or break anything.

Hope this helps, best of luck. :drinks:

[Marlin Junky]

MJ,
If I'm reading your response right, you may have read Leftiye wrong.

I'm pretty sure Leftiye was talking about a step shank config similar to those appearing on .458" designs from BRP:

http://www.brp.castpics.net/Rifle%20Molds.html

[sagacious]

Yes, I agree. The LEE 452-300-RF has exactly that feature. In my tests, recovered bullets still had nothing but lube in that groove; no fouling or lead shavings at all. So, it's functional utility is far from conclusively shown, or even adequately demonstrated, as I mentioned above.

Thus, an extended shank that forms an incidental groove vs a shank with an intentional partial groove? No observable difference in my tests. I thought that was what you were after. If I read you wrong, my apologies.

Good luck. :drinks:

[runfiverun]

anything in that groove would/should be tossed out after exiting the bbl.

[sagacious]

anything in that groove would/should be tossed out after exiting the bbl.
In my tests it was still full of lube. And I use a soft lube.

Sometimes 'em little buggers don't do what one thinks they should. Go figure.

[44man]

Right, but if a groove can't be created in front of the seated check because of machining limitations, are there any down sides to creating a straight shank that's somewhat over 1/10"? Say, 1/8"?

Thanks for the response... I still need PB input.

MJ
I have made all kinds of molds and one of my most accurate PB has a base band only .075" long. Others are .100" and I can't tell any difference. I have read a ton of stuff on boolit design where you need a long front band and a long rear band, etc, but have not found any real solution.
Now these are for my .475 but it also holds true for my other calibers, I just have a better picture of these.
The two left boolits are the most accurate, the third is not good at all and the Lee on the right shoots pretty good. The target at 50 yards was shot with the left boolit and the can was shot twice at 100 yards with it.
I have no answer for you except to make the boolit look good.

[Bass Ackward]

There are two factors to consider / overcome.

As a bullet leaves the control of the muzzle and the bullet is still under control, two things must happen. The nose must break the barrier and establish an air flow pattern. The second is that the bullet must withstand the rotational forces of the rifling on the base.

Considering the second aspect, the wider that band is, the stronger and more flexibility you will have for hardness of your slug.

This was where the old theory on faster powders with cast came into play. Have the bullet accelerate quickly and pressure drop rapidly so that the bullet is up to full rotation when it exits instead of still be accelerating. That strategy puts the least amount of stress on the back band. Plus there is less muzzle pressure to do bad things if everything is NOT perfect upon launch.

Velocity through air becomes the real enemy creating limits for a metal with only limited strength. So obviously you can see that aerodynamics comes into play especially from a flexibility standpoint. If you can launch it well, it will fly well.

There is one more major factor to consider on rear band width after the launch and that is bullet flight from the location of the center of balance. Full diameter lead is the heaviest factor to a bullets weight, so the wider a back band is, moves the CofB. If it goes in the direction that you expect, then good things will happen for the velocity range of interest. If it doesn't then you will have a poorer performer from an accuracy standpoint within the velocity range you insist on using it.

The real question is how flexible do you want this design to be?

[44man]

I shoot pretty hard Bass. That 420 gr boolit is going 1330 fps. The gas check boolit shown actually shoots better with 1/2 gr more powder, been down to 1/2" at 50 yards but it sticks brass.
ALL of my boolits show wide land marks at the front but will be perfect at the base. Just need to fiddle with the alloy a little but even water dropped WW metal gives me no trouble. Accuracy and no bore leading.
When I finished the mold I looked at the base band myself and wondered why I did that. I never drew the boolit on paper, I just kept running the GG cutter in where it looked good. The nose was shaped with a file until I liked it.
It has turned into my premier target and deer boolit.
I gave up the heavy thinking long ago and if a boolit doesn't shoot, I throw the mold in the back of the drawer. :mrgreen:
I would be jittery if I had to pay for a mold!

[Marlin Junky]

Seems to me that the boolit base would need to be strong enough to withstand the rotational forces yet soft enough to obturate when engraved by the lands. Again, my interest is in 44 and 45 caliber rifles that typically have 20" twists (Marlin 336s and their variants). Currently, I'm experimenting with a 444 and a 305 grain PB (custom MM) with a base band that's about 5/32" long. My launch speed with 4759 is 1600+ fps and my alloy is pretty hard (I'll measure it tonight, but I'm pretty sure it's over BHN 15. I'm sizing to .433" and lubing with BAC. This load puts rounds right next to each other at 50 yards (w/ receiver sights) but produces an occasional flyer. After 12 rounds there are a few flecks of alloy on the wet cleaning patches but I don't notice any roughness near the chamber when cleaning. I'm trying to decide if I were to have another mold made for this gun if I should make the trailing band (or base) shorter. Perhaps it be wise to experiment with a softer alloy first. Maybe some of you guys that have lots of experience with the 45-70 can indicate what PB boolit mold designs have worked best for you when shot fast.

Thanks,
MJ

MJ

[Bass Ackward]

This load puts rounds right next to each other at 50 yards (w/ receiver sights) but produces an occasional flyer. MJ


You said it correctly. That "load", not the bullet design.

The problem with TOO wide of a band is having enough lube left for the bands departure. Since you are already flecking, I would add a coat of LLA first and see what happens.

[44man]

Seems to me that the boolit base would need to be strong enough to withstand the rotational forces yet soft enough to obturate when engraved by the lands. Again, my interest is in 44 and 45 caliber rifles that typically have 20" twists (Marlin 336s and their variants). Currently, I'm experimenting with a 444 and a 305 grain PB (custom MM) with a base band that's about 5/32" long. My launch speed with 4759 is 1600+ fps and my alloy is pretty hard (I'll measure it tonight, but I'm pretty sure it's over BHN 15. I'm sizing to .433" and lubing with BAC. This load puts rounds right next to each other at 50 yards (w/ receiver sights) but produces an occasional flyer. After 12 rounds there are a few flecks of alloy on the wet cleaning patches but I don't notice any roughness near the chamber when cleaning. I'm trying to decide if I were to have another mold made for this gun if I should make the trailing band (or base) shorter. Perhaps it be wise to experiment with a softer alloy first. Maybe some of you guys that have lots of experience with the 45-70 can indicate what PB boolit mold designs have worked best for you when shot fast.

Thanks,
MJ

MJ
You have a much wider variation with hardness in a rifle, something I don't have with a revolver. As long as you don't start stripping the rifling with the whole boolit you are good to go.
I made a PB boolit for my 45-70 BFR the same way I did the others. I just cut GG's where it looked good. It has 2 GG's, front band is .140", base band is .145", meplat is .370, weighs 385 gr and is 22 BHN. So you could say it is almost a WFN.
I shoot it over 1600 fps and it is supremely accurate.
But it's failing is that it just pokes a boolit size hole through deer with that much velocity and leaves just a hole in lungs. My last deer with it went 200 yards.
I can't shoot softer lead from the gun without shots going all over the paper so to make it work I would need a soft nose. Now from a rifle, this boolit would work great with a softer alloy.
Not only that but your 1 in 20" twist would be easier on the boolit then the fast twist I have in the BFR. Mine is 1 in 14". One of the reasons I need a hard boolit. The BFR has a long run to the forcing cone then the boolit has to be spun up in the fast twist.
The whole truth about the gun is that it has been the most accurate and easy to work with revolver I ever owned as long as I shoot harder boolits and it doesn't matter much which boolit or weight from 300 to 420 gr. The fast twist was a real eye opener.
Bass is right in that if you just pick a decent boolit, you can find a load even if you experiment with alloys. Your choice comes down to what boolits operate in the Marlin action.
You can use slower powders then I have to, to extend the pressure curve which also is easier on the starting boolit.
Forget the base obturation stuff! If a boolit skids past the base, pressure will not fill the gaps by forcing lead into them, gas is going through the gaps and making them worse. It is like trying to plug a high pressure water leak with a wad of bubble gum. Think the gum would seal the boolit with 30,000 psi? OH, I forgot, lube is supposed to do that! :kidding:
The key to accuracy is prevention first. There is not a single stop gap measure that works in a gun.