question about gas check cast projectiles.

[opos]

I had a question concerning use of gas check projectiles in a rifle. I have been told that a cast boolit that is 1 or 2 thousanths of an inch larger in diameter than the bore (as determined by slugging the bore) should provide a proper seal and give good results as a starting point in determining loads.

If a cast projectile uses a gas check does the gas check also expand to help seal the chamber when the boolit is fired? I can understand a non check projectile expanding and obturating to seal the bore but am puzzled if the same process takes place if the projectile is gas checked with a copper check...I'd think the check would stand in the way of the obturation and interfere with the action of the boolit as it's fired.

Hope this makes some sort of sense....I have a Mosin that slugs at just about .313 and have found a commercial caster that offers a 180 grain gas checked projectile that is .314. Sounds like a decent way to go (I'm disabled and casting is not feasible for me to do)...but if the gas check holds the base of the projectile it seems like it won't obturate to fill the barrel...any input really appreciated
opos

[giericd]

I'm no expert and new to casting but I would think (but could be way off) that the gas check does little in the way of a seal. I think bullet profile has more of an impact such as a flat based bullet would make a better seal than a boat tail bullet. Gas checks in my opinion serve only one purpose and that is to help protect the base of the bullet from the intense heat when the round is fired which reduces splatter and leading which also would affect accuracy.. My understanding is that most gas checks come off as they exit the barrel, that would tell me that crimping the gas check on just prevents it from falling off being loose between the powder and base of the bullet. I can't see in any way that a pice of metal that is only .005-.007" thick which is held on by a crimp is going to twist with the bullet and fitting tight enough to make a seal and be come groved from the rifling like the jacket would on a FMJ round, no crimp is strong enough to withstand that torque with 20-45,000psi behind it. Just my opinion. I would think you should get a pretty good seal if your bore is .313 and the bullet is .314 seems to work fine for the .223 which takes a .224

[giericd]

Also I would think you would get a slight expansion in the midsection of the bullet when it is fired regardless if it was checked or not. With that much pressure in the case the rear of the bullet would have to start moving forward (checked or not) a millisecond before the front of the bullet started to move causing the slight swell in the midsection. Check out a video on an arrow being shot in slow motion and see how much flex the arrow has. Sorta the same thing.

[Bigslug]

Think of your boolit as a car that is parked closely behind a second car (the throat of your barrel), when it gets rear-ended by a third car (the pressure wave of your powder). To a degree, the middle car (your boolit) is going to expand in directions not blocked by other automobiles (toward filling the voids between it and the barrel walls). A gas check is not likely to change this much, if at all.

[giericd]

Happy to see that my line of thinking even being new to this is inline with what a boolit master thinks! At least I know I'm not going crazy or burning brain cells thinking about this subject for nothing.

[leadman]

The gas check before seating is larger than the bore size as the sides are tapered outward. It is crimped in place by the sizing die so if the die is .314" the gas check is usually around .3145" as there is more springback of the copper or aluminum check than there is of the lead.
Yesterday I shot a rock in the desert with a Lyman 314299 boolit, 200grs, sized .314", out of a Mosin Nagant rifle. The rock broke but I did find what was left of the boolit. The gas check was still intact on the base of the boolit with about 1/4" of bore sized lead with some expanded lead still hanging on. There were rifling grooves in the gas check.
If this little piece of metal did not stop gas do you think it would be called a gas check?
Pound a boolit down the muzzle and knock it back out and take a look at the gas check.

I went and found that piece of boolit and took some pictures, hope you can make them out. This boolit was .314" to begin with and was shot down a bore that is almost .316". No leading after 30 rounds with the Hi-Tek coating.

[Scharfschuetze]

I have recovered numerous gas checked rifle boolits from snow banks and soft earth and the crimp on gas checks that I like to use have always remained firmly attached after exiting the muzzle and impacting the backstop. The gas checks also indicated that they had fully "checked" the gas at the base of the bullet and they were fully engraved by the rifling of the barrel. When inspecting the bullets, there was no evidence of gas getting past the gas checks with boolits sized one to two thousands over the bore diameter.

In regards to the GC interfering with boolit obteration, I think that the light copper gas check getting pushed into the heavier (inertia resistant) boolit would in no way hinder the lead alloy from obterating.

The only gas checks that I've had come off was with the older Lyman GCs that didn't crimp on. I think that the crimp on type gas checks receive further "crimping" on their way up the bore and is one of the reasons they remain so well attached. In my experience, GCs create a good barrier to the expanding high pressure gas behind them when sized slightly over bore diameter as is the norm for most cast boolit shooting.

[44man]

Here we go again! the GC is a skid stop ONLY! BOOLIT BASES ARE NOT HARMED BY HEAT OR POWDER.
Cast can skid rifling opening gas ports so the check was designed to halt skid and keep the seal at the base. Use real soft lead and the check will also fail.
Checks are NOT larger then what you size at, they are tougher then the lead to grab rifling.

[leadman]

I disagree that checks are not larger than sized at, at least for 30 caliber rifle. Unless annealed the copper Gator checks do spring back a little and I have measured many or them. The aluminum I buy from 338 Ultrmag here also springback a little. I'll have to measure some Hornady checks to see if they do the same thing.
There was no mention that gas sealing was the only thing that gas checks performed while in the bore.

[Larry Gibson]

Scharfschuetzer nailed it.

Was shooting yesterday helping a friend develop a 314299 load (cast at .3155 and just lubed and GC'd) for his No 4, Mk I .303. He was using aluminum GCs he had got off the internet. They only gave a slip fit on the GC shank and could be pushed off relatively easy. All was fine until around 1800 fps when accuracy went sound. The holes in the target show definite evidence the GCs were coming off. Had the same problem with those GCs on the GB 314291 in that rifle. When we used the Hornady brass crimp on GCs or my own brass GCs that crimp on accuracy was excellent up through 2000 fps. He really wanted to use those GCs as he bought a lot of them...........sometimes what we "want" just isn't going to work........so he's buying some Hornady GCs now.

Larry Gibson

[DougGuy]

Basic rules about gas checks.. (correct me if I am wrong, but this is my understanding of gas checks and their fit/function)

1. Gas checks do NOT come off when exiting the barrel like a shotgun wad. Unless they are poorly made or improperly installed.
2. Gas checks improve accuracy by effecting a more efficient seal in the rifling than a cast boolit alone.
3. Gas checks allow a somewhat softer alloy to be used which in hunting situations will afford more expansion.
4. Gas checks will NOT de-lead a barrel. They will skip right over the lead in the grooves leading to poor accuracy and more leading.
5. Gas checks just like any other boolit will spring back slightly unless annealed to dead soft.

For my uses, copper gas checks just work. They were a brilliant invention when conceived, and used properly are excellent components to use with cast boolits.

[DLCTEX]

The check helps prevent gas cutting of the boolit. If your boolit is over bore size there is no obturation of the boolit as there is no room for expansion, no is any obturation needed.

[mdi]

I'm glad to read Misters Scharfschuetzer and Gibson replies. In my limited experience/thinking (life long machinist/mechanic) their answers coincide with my thinking, although the check gripping the rifling as per Mr. 44 is very interesting. I don't know that I've ever had any cast bullet skidding take place as recovered bullets show the rifling very plain. But one thought; if a bullet skids and the check grabs the rifling, why would the bullet not spin in check, or the check spin with the rifling and the bullet skid and not spin? Is the crimp enough to keep the bullet/check in one piece? Just thinkin'....

[454PB]

Gas checks DO remove leading. I was experimenting with a dry lube using my Ruger RH .44 magnum a few years ago. The lube failed miserably and completely coated the bore with lead, to the point that it was hard to see any rifling. I fired a cylinder of 429244 boolits with gas checks installed, and when I later cleaned the gun, there was not a hint of leading.

[Larry Gibson]

I also am of the opinion that GC can and do remove (scrape?) some fouling, excess lube and leading from the bore. However to be effective at that there must be a tight fit between the GC shank, the GC and the lands and grooves. There also must be space between the last drive band and the front of the GC for the fouling to accumulate. They do not remove heavy leading and fouling though.

Larry Gibson

[Char-Gar]

I can tell you what a gas check does, but I won't attempt to tell you why it does it. When we get to the why, folks start to line up with their pet theory which they try to pass off as fact.

Here is the what:

1. A crimp on gas check won't come off in flight or with impact.
2. A gas check will allow you to push the bullet much faster than a plain base bullet with accuracy and no leading.
3. Plain base bullets pretty much loose their utility much past 1,400 fps.
4. A gas check bullet can be pushed (with the right alloy, size, lube etc. etc.) well past the 2,000 fps mark before the accuracy starts to roll snake eyes.

I have done some reading on the development of the gas check and like today, I come away with different views;

1. Some believe it was designed to be a driving band.
2. Others believe it was designed to protect the bullet base from hot gases.

Whether by design and intent or just by accident, the gas check does some very good things for a cast bullet. They do add to the cost of the bullet and if the needed additional performance is not needed, they are no a good use of your reloading dollar.

Like others, I hold some opinions about why gas checks do what they do, but I don't have either the education, equipment nor inclination to prove these theories to a valid scientific conclusion. Without that, they are just opinions and I prefer to keep them to myself. I have grown very tired of the bickering and opinion mongering on this board and prefer to not join in that kind of fun.

There are quite a few folks (myself among them) that can tell you how to make cast bullet preform like a show pony in your rifle. About the same number think they know why things happen like they do. But, a far far smaller number actually, really and truly know why things happen like they do. I have already confessed that I do not.

[leadman]

I had leaded the bore in my 357 years ago and tried to delead it with the Saeco 180gr RFN GC. Did not work as there is not open area on the boolit between the front (or top) of the gc and driving band. The Lyman 160gr SWC with a space in front of the gc did remove most of the lead.
The aluminum gcs our member mentioned earlier have worked as well for me as the Gator checks and so far as the Hornady checks.
I think that the lead driving bands engraving the rifling give enough resistance to allow the pressure from firing the gun to adhere the gc to the lead base, along with the crimp. How hard would it be to turn one steel plate on top of another with no pressure on it, then put 20K of pressure on it in a hydraulic press and try to turn it.
There must be some obturation of the gc in the bore as the photos of the piece of boolit show the rifling has engraved the gc, yet this boolit was about .001" smaller than the bore and .0015" smaller than the throat. Started at 16grs of SR4759 and worked in 1 gr increments to 20grs. At 18grs the groups shrunk to less than 1" and opened up a little at 20grs. This was a 57 yards. First shooting session with this rifle and had not prepared any boolits specially for this gun. I now have some that measure just under .316" and hope to try these next week.

[felix]

.... if a bullet skids and the check grabs the rifling, why would the bullet not spin in check, or the check spin with the rifling and the bullet skid and not spin? Is the crimp enough to keep the bullet/check in one piece? ....

Another reason the slowest twist with the longest leade wins! There will always be a tendency for the boolit to spin within the check, because the boolit's front always twists before the boolit's end during complete boolit entry into the barrel. Boolits upon recovery without frontal damage will typically show that skid, especially when the boolit is long and heavy. What matters is that the boolit's tail end for about 20 percent or more up towards the front mirrors the lands of the barrel exactly. If not, the boolit's RPM is not theoretically accurate between individual shots and the grouping will be terrible. ... felix

[cbrick]

I have been told that a cast boolit that is 1 or 2 thousanths of an inch larger in diameter than the bore (as determined by slugging the bore) should provide a proper seal and give good results as a starting point in determining loads.

If a cast projectile uses a gas check does the gas check also expand to help seal the chamber when the boolit is fired?

I have a Mosin that slugs at just about .313 and have found a commercial caster that offers a 180 grain gas checked projectile that is .314 but if the gas check holds the base of the projectile it seems like it won't obturate to fill the barrel. opos

This thread rapidly turned into "what does a gas check do". Interesting but that wasn't the OP's question.

The answer to the original question is, the gas check will be no smaller than the sized diameter of the boolit (because the sizer die that sized the boolit also sized the check) plus probably a bit of spring back. Your bore slugs at .313" . . . Your boolits (including the checks) are .314" so there is nothing to obturate and both boolit and gas check will be swaged down to groove diameter entering the bore. That's why you use a boolit .001"-.002" over groove diameter, no obturation needed or possible.

Obturation (increasing the diameter) would only be possible for the boolit or the check IF either were smaller than groove diameter to begin with.

Rick

[Dan Cash]

Here we go again! the GC is a skid stop ONLY! BOOLIT BASES ARE NOT HARMED BY HEAT OR POWDER.
Cast can skid rifling opening gas ports so the check was designed to halt skid and keep the seal at the base. Use real soft lead and the check will also fail.
Checks are NOT larger then what you size at, they are tougher then the lead to grab rifling.
I beg to differ. You need to read "The Bullet's Flight" by F.W. Mann wherein many photographs and descriptions of propellant damaged bullet bases are to be found.

[opos]

Well I learned a lot about gas checks...I've also had a couple of p/m's about the subject and all have been very helpful to me...I know which way I'm going to go and that will be a good starting point....I shoot a lot of 12 hardness RNFP in my 45 colts over Trail Boss for soft "arthritis proof" loads....same with my 357mags...but figured that in the Mosin (and probably in my 30-06 Winchester Enfield and my 8mm Mauser) a bit harder (18) gas checked lead projectile that is gas checked and about .001 over the slugged diameter would be a good start point. Got a lot of Varget and for now will just play with moderate loads using that and the commercial cast/gas checked projectiles I've settled on to try.

Want to thank everyone again for the input and comments..been a big help

[felix]

Jim, boolit bases DO get hammered by the propellent kernels and/or by the gas developed. The boolit base will be concave and/or splattered when propelled hard/quick enough, checked or not. I've seen it too many times, so I am sure. ... felix

[mdi]

Keep it up, fellers. I'm getting a bunch of good info...:popcorn:

[weasel 21]

I just seated some 30 cal. GC ; then pan lubed the bullets. They formed a beautiful as cast lubed bullet. Now do I have to scrape the lube between the last band & the GC?

[leadman]

The lube is fine there as long as the boolit does not lead the bore. When I lube in a RCBS luber/sizer this gets filled in with no issues upon firing.