Theoretical question on gas checks.

[Mik]

I woke up at 3am with an idea about gas checks, and my wife didn't want to hear it, so I figured I would post it here. The idea involves some pretty significant assumptions developed by the inexperienced casting/loading part of my brain, so feel free to correct me.

Assumptions:
1. A major function of the gas check is to serve as an engagement surface for the rifling. This surface prevents the soft lead projectile from "pushing through" the lands which leaves you with a deformed projectile, a leaded barrel, and an oversized groove in the projectile which may allow for gas cutting.

2. The base of a plain lead bullet is not exposed to heat and pressure long enough to melt on its way down the bore.

Question:

Why do gas checks ride at the base of the bullet? It seems to me (if the assumptions are correct) it would be beneficial to have the engagement surface as far forward as possible. This would eliminate or reduce the deformation caused by the lands before the gas check engages the rifling. Would it be possible to manufacture a cast bullet with a "gas ring" in place of the first driving band?

[357shooter]

A big benefit is the ability to shoot soft lead bullets, that expand well, out of rifles. The check prevents the soft lead from obturating or causing undue deformation and over pressure. The benefit is negated with a soft lead base and bands being exposed. Besides, they way to achieve what you suggest is to wrap it around the bullet. If you do that you just end up with a jacketed bullet, or semi-jacketed bullet.

Your option 2 is incorrect BTW.

[Mik]

I wasn't thinking a jacket, more like a small ring that you would slide over the nose of the bullet before sizing like this (excuse the artwork):



But, if assumption 2 is just flat out incorrect...that answers the question about the location of the gas check and this idea doesn't make sense.

I thought there were a few people on this forum who had done some testing and found the base of the bullet doesn't actually melt enough to cause accuracy problems. That is why you can use fast powders to shoot pb loads in a rifle at 1200 fps even though the heat and pressure peak may be similar to 2000 fps loads with slower powders.

I'm probably wrong. Again, just a 3am thought.

[GhostHawk]

Wilkes gas check, was posted here just a few days ago.

Basicly a gas check with a hole that could be placed in the mold, then pour in the lead.
I suspect it was a pain to do for more than a few bullets, and it has to have a near perfect fit with the mold or you have problems.

On the plus side from what I read, it could indeed improve accuracy.

http://castboolits.gunloads.com/show...ade-gas-checks

[JeffinNZ]

They are on the base cos that is the easiest place to put them. Also, the primary function to prevent gas cutting at the top end but some extra grip does not go a miss. Case in point; my .30-30 will shoot 40-1 alloy at 1750fps VERY accurately with copper checks but not with alum.

[Sagebrush7]

Jeff, Why? Beer cans making the bullet stumble? I bet it is the alloy of the metal you are using.

They are on the base cos that is the easiest place to put them. Also, the primary function to prevent gas cutting at the top end but some extra grip does not go a miss. Case in point; my .30-30 will shoot 40-1 alloy at 1750fps VERY accurately with copper checks but not with alum.

[JeffinNZ]

Jeff, Why? Beer cans making the bullet stumble? I bet it is the alloy of the metal you are using.

Yeap. That's VERY soft lead alloy for that sort of FPS. Alum checks are Yonky's 3XXX series. I firmly believe that the 40-1 alloy at that FPS and pressure needs a spot more traction.

Cu left, Al right.

[EDG]

In the late 1960s there was an exceptional article in the American Rifleman about cast bullets. One of the items briefly discussed was installation of copper tubing rings in a bullet mold before the bullet was cast. Perhaps the idea could be developed more. However it probably makes casting more difficult and I do not know if there are any real benefits.

I wasn't thinking a jacket, more like a small ring that you would slide over the nose of the bullet before sizing like this (excuse the artwork):



But, if assumption 2 is just flat out incorrect...that answers the question about the location of the gas check and this idea doesn't make sense.

I thought there were a few people on this forum who had done some testing and found the base of the bullet doesn't actually melt enough to cause accuracy problems. That is why you can use fast powders to shoot pb loads in a rifle at 1200 fps even though the heat and pressure peak may be similar to 2000 fps loads with slower powders.

I'm probably wrong. Again, just a 3am thought.

[357shooter]

Sorry about my comment on item 2. I read it backwards I think. The bullet base doesn't melt, even with soft lead. It's the gas cutting and the deformation caused by the lands that cause the issue.

The big benefits of the checks are the grip and twist effective they have, along with not letting the base of a soft bullet create any overpressure situation.

It can depend on the caliber and rifle too. My 357 lever action will shoot plain base BHN 8-10 cast at 1600FPS all day with no leading and no over-pressure. The pressures involved are a lot less then most rifle calibers so this may not hold true for them.

[historicfirearms]

Another benefit to putting them on the base is that the rifling doesn't extrude lead rearward, leaving a much more uniform boolit base. (See your 1. Assumption). A uniform boolit base is just as important to accuracy as a uniform muzzle crown.

[JSnover]

That would be a driving band, like you find on artillery projectiles. It's an idea that has been tried a few times with cast boolits. The article I read was called "375 Rook Rifle" or something like that. The author cut copper tubing into rings and put them in his mold before casting.
He reported good accuracy with reduced loads and no leading.

[Mik]

Thanks for the info everyone. The post about the gas check helping to maintain a uniform bullet base is a great point and not something I had considered.

I wonder if there is any way to get this ring onto the bullet as part of the sizing process instead of the casting process. I'm guessing, at the very least, you would need a mold that leaves a "shank" where the lead driving band would be on most designs. Then maybe you could crimp the ring in place with the sizing die.

At any rate, it must not be that much better than a base check because its been tried, worked, and hasn't caught on.

[Coffeecup]

Mik-I tried that after burning my fingers too many times whilst experimenting with Wilke checks. The biggest problem with applying the driving bands before sizing was that even when the bands were annealed, there was some spring-back from the sized diameter.

It is very similar to sizing down a jacketed bullet in a CB sizing die. The jacket (or driving band) sized down to die diameter, then springs back to a larger diameter. Meantime, the lead has been reduced in diameter with less (or no) spring-back. You wind up with loose driving bands, which never seemed to work well for me.

Now if you could apply the driving bands then bump the bullet (and driving bands) up to a larger diameter, you might have a better system.

[a.squibload]

How much would a copper ring expand when heated?
Heat ring, install, shrinks to fit?

Another reason for copper rings on cannon projectiles is to keep the steel
off the bore.

[OuchHot!]

As EDG points out, there was experimentation along those lines. One company sold zinc washers that were placed in the lube groove of choice in the mold. It was a real pita but did work as a lead scraper. It was just too much trouble. Later folks worked with Cu washers and thimbles in swedging dies to do the same thing. I played with Cu washers and tubing thimbles placed in the mold. I don't recall any results that justified the trouble in my efforts, but they were rudimentary.

[chief3]

Many years ago (60 ?) I saw in a book by Elmer Keith a series a high speed photos that showed the base of a cast bullet being upset by powder grains just as the bullet left the muzzle. Could it be one more thing gas checks take care of ?