View Full Version : New GC use?
12-23-2007, 08:32 AM
Does anyone have any idea, or done any work ont the theory that a GC actually acts as a "semi-jacket"? That is does the extra strength of the gas check help keep the boolit stable and keep it from stripping in the rifling?
Over the past couple of years I have been working with a Microgroove Marlin 336 in 30-30 and it seems any .308"-.311" boolit I put in it shoots well so long as it has a gas check. Velocities range from 1500 fps to 1900 fps.
Lubes have been LLA, my own concoction, and manufactures own lubes.
Boolits have been Laser-Cast 170's, Lee 170 grain and 150 grain, and Lee C312 185 grain.
Alloys have run the gamut from plain lead to WW to 1/3 lino with 2/3 lead.
Powders have been Unique, Red Dot, AA-5744, IMR-3031, Winchester 748.
Primers were WLR or Federal GM210M.
They have all shot well, but only with a GC.
The only PB boolit I have tried is the Laser-Cast 160 grain and they shot patterns, not groups.
I just wonder if the GC has strength enough to grab the boolit and the rifling and keep everything stable enough to shoot well.
Any thoughts on the matter?
I'm with you on the findings. After playing with two different original 38-55s (the ones with the under-sized chambers and over-sized bores :() it may be that the GC has had that effect all along but has primarily been credited with only the one- protecting the base. There is no doubt in my mind that indeed it protects the base but also keeps gas from cutting up the bullet shank thus preserving the integrity between the rifling and the bearing bands on the shank. The GC may also have the added benefit of "taking" some of the start-from-zero bullet rotational inertia thus aiding in the rest of the bullet's bearing surfaces ability to take the start-from-zero rotational inertia during acceleration. In short :mrgreen: yes I'd tend to agree with your theory. Just my opinion- don't want to step on any anecdotal, golden calf toes here :)
12-23-2007, 01:42 PM
The GC may also have the added benefit of "taking" some of the start-from-zero bullet rotational inertia thus aiding in the rest of the bullet's bearing surfaces ability to take the start-from-zero rotational inertia during acceleration.
Sounds reasonable to me.
12-23-2007, 02:57 PM
I dunno. The GC is waaaaay down in the case when the boolit hits starts turning. Be kind of hard to say it takes the rifling when it isn't even touched by the rifling. Not that anyone is right or wrong, just thinking out loud so to speak. Greater minds than mine inhabit these environs, and hopefully they'll check in.
As for the lack of success w/o GC's- that can be any number of things- better ignition, resistance to gas erosion, better base formation on exiting the muzzle... buncha other stuff I'm sure I haven't thought of. Truth is some guns just react better to GC designs than PB. Or at least it's way easier to get good results. Just my two cents.
Correct. Obviously, the front bearing band in a normal cast bullet design will engage the rifling first. But in a rifle, between the time the powder gas starts moving the bullet out of the neck and the time the GC engages the rifling the bullet is going no where near the normal MV of say 1000-2000 fps, it is still very near the front portion of the acceleration curve and it is no where near its final spin rate of some 100,000-200,000 RPM. I think the original idea that Lead melter brought up is valid and may have been mostly overlooked in the past with descriptions of the function of the GC. Exactly what % of the rotational inertia load the GC carries as it relates to the bearing band load.... who knows?? Probably depends upon such things as bullet diameter in relation to bore/groove diameter, dimensions of lands, hardness of bullet, surface area of bearing bands, weight & SD of bullet, type and amount of powder and acceleration curve of particular bullet with that powder, etc. Regardless... an interesting topic. :)
12-23-2007, 04:52 PM
With the old factory-loaded soft swaged 240 grain lead semiwadcutter gas check .44 Magnums, I recovered some bullets back in the early '70s fired from my Super Blackhawk that showed the rifling grooves on the bullets (from the lands) that were clearly wider at the front, tapering back to the gas check, but the grooves in the gas check were sharp and just as wide as the lands. I thought then that gripping the rifling was part of the benefit of a gas check. Maybe one of these days I can recover some of my soft cast #429244HPs (or the Ranch Dog when I get it) intact enough to check. All I get back out of backstop dirt is a lead button with the gas check on it, at best.
12-23-2007, 07:46 PM
I don't believe that a crimp on gas check gas check will prevent a bullet from stripping the rifling. A crimp on g.c. can be turned on the bullet g.c. shank with a pair of pliers in most instances.
I ran a test several years ago between the old Lyman brass gas checks that fit somewhat loose on the g.c. shank and the Hornady crimp on types. 150 gr. .30 caliber cast at 2,300-2,400 f.p.s. Alloy: Linotype Barrel twist: 1/10
Result: No discernable difference between the two.
12-23-2007, 07:54 PM
I agree 100% on your findings. I've had the same experiences with GC'd bullets in the Marlin 336 MG .30/30.
The GC does in fact grip the rifling and tightly also.
We were shooting steel plates one time at 100 yards. I had the only .30/30 and it had been rainning so we spent a good moring banging steel plates.
After the shooting, we were looking at the plates and noticed fully spread GCs in front of the plates from my bullets which were #316 Saecos.
The CGs had fully expanded. There was a .30 circle in the middle and the outer ring was deeply engraved by the micro grooves. So, the rifling does indeed grip the GC and in teh case of the MG rifling enhances the grip placed on the bullet./beagle
12-23-2007, 08:14 PM
Yes, the gas check is gripped tightly by the rifling, but the bullet is "plastic" underneath the g.c. and since the grip area of the g.c. is small, it is somewhat doubtful that it adds much at all to the bullet grip.
One way to test that theory would be to fire a blopper load, retrieve the bullet, place the gas check into the muzzle and see how much force it takes to turn the bullet.
However, in a full power load, obturation could "upset the apple cart" in the above theory. :coffee: :-D
12-23-2007, 10:06 PM
Another thing to look at to is that bullet is going to expand under high PSI and make that gas check even tighter on the bullet and a PSI of 40-50000 pushing on the back of it i dont think its going to spin much on the bullet. Thats just my thought. :)
12-24-2007, 03:52 AM
I would say it does. At least all the little things that I do with straight shanks to annealing checks seems to raise top end results. I get far better HV results with straight shanks than tapered.
And I don't believe that all cast bullets go plastic until you have exceeded the yield strength of the metal. Especially if it takes a certain pressure just to begin obturation.
12-24-2007, 09:22 AM
Lotta food for thought in this thread. The only rifle calibers in which I've had success with plain-based bullets are low-pressure critters like the 44-40 WCF (in '73 Winchester) and 45-70 with 1873-level pressures/velocities. I'm not sure what combination of factors is at play here, but the elements mentioned herein so far certainly seem to influence the outcome.
In my own work, there seems little difference between rifle-length and pistol-length barrels--both start "needing" gas checks at pressures that create about 1350-1400 FPS. More clearly--accuracy falls off with plain-based boolits at those speeds. Since barrel length seems to not greatly influence the "need" for gas checks to maintain accuracy........I tend to think that whatever "good" or "bad" things occur have their roots very early in the boolit's travel up the bore.
As always--keep in mind that all this comes from a social science major in college.
12-24-2007, 12:39 PM
Let me throw another bee into the proverbial bonnet.
Since the process of obturation is simply to deform the base of the boolit to the point at which it seals the bore preventing gas blow-by, could it be that the higher tensile strength of the GC causes obturation in another part of the boolit farther along its axis toward the nose?
This pressure induced condition should, and I say should, create a situation where the potential accuracy of the boolit is increased, since there may now be two or more portions of the boolit sealing the bore and keeping its alignment steady. This situation is assuming that all other factors including boolit fit, alignment, etc. are all correct.
Now, if we assume also that the base of the boolit becomes somewhat fluid while traveling done the bore. This scenario would allow the GC to come into an almost perfect perpendicular ratio with the boolit base, thereby allowing the boolit to exit the barrel squarely, and thus more stable.
I relenquish the floor to the next speaker.
12-24-2007, 01:05 PM
You covered it quite well. Nothing more to add. ... felix
12-24-2007, 10:40 PM
Unless you want to work "paper patching" into the argument. :-D
12-25-2007, 01:42 AM
I added one bee...you're trying to move the whole hive!
12-25-2007, 04:46 AM
Bless you Mark for bringing up BEES, interesting read....One could put a timming mark on the bullet and the GC to prove/disprove any tourque theories,providing the mark would stay through umteen thousands of heat and lbs of pressure.
12-25-2007, 05:00 AM
I think the main benefit of a GC is to prevent the edge of the bullet from melting.
Lead melter and Felix, Yes! Sounds right and agree.
I think obturation has to do with the resistance to accleration by the inertia of the bullet within a confined space whereby the greatest amount would be at the rear and the least at the front. Would seem in bullets with a lot of mass ahead of the front bearing band (large nose) there would be more obturation at the front band than in those with lesser mass ahead of the front band (small nose).
The obturation at the GC would likely be stopped by the confines of the bore..... but the radial potential for obturation may increase slightly just ahead of the GC because of harder nature of the GC. Also, at normal pressures say 10,000 psi and higher would not seem likely that a GC could spin freely on the bullet base because: obturation into the lands would crimp the GC onto the base and the friction caused by the pressure exerted on the GC by the gas charge behind it.
Too cold outside to shoot comfortably so here we sit contorting CBs, physics and the ballistique :mrgreen:
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