The real story on Gas Checks

[thegreatdane]

The picture I promised.

[TXGunNut]

OK, just got my first GC mould and read this whole thread. Wow! All the physics and metallurgy was fascinating so I dumbed it down enough so that I could understand it; please correct me if I'm wrong.
1. GC boolits perform almost as well as j-word bullets in the bore.
2. GC boolits perform as well as cast boolits in the boar.
3. GC boolits get thru the bore faster and cleaner and hit harder just in case the boar is big and mean(er).

All kidding aside, you folks are awesome! I've hesitated to get into GC's but they're on my project list, might as well start thinking about it. I think I'm ready to give it a try...very soon!

[Old Kid]

I'm new here, although I've been reading posts for some time. I'm casting various calibers of boolits and have been experimenting with gas checks. From what I've read here it seems that I can use softer alloy when I'm using a gas check and still not have to worry about leading. Is that the case?

Old Kid

[Molly]

Hi Old Kid,

In my experience, the improved performance from gas checks is because they are much higher in melting point than lead alloys, and hardness doesn't seem to have much to do with it. As the round is fired, there is some gas leakage along the base and sides of the bullet. These gases are very hot, and will etch a very fine spray of molten lead off of the bullet base edges and the sides of the bullet. This molten lead is then soldered to the sides of the bore as leading when the bullet catches up and passes the spray. Gas checks reduce this because they don't melt or etch nearly as easily as lead. But they're not perfect, because as you increase you load, the gas can and will etch more and more droplets off of the side of the bullet, and you will eventually get leading again.

There IS a simple, easy and inexpensive way to prevent gas leakage entirely, and allow you to shoot cast bullets with jacketed bullet loads. It works with all loads, from power levels suitable for everything from mice to moose. Just place a little bit of Cream of Wheat ("COW") beneath the bullet. This will let you shoot cast bullets just as they drop from the mold: No gas check and no lube. And if your throat is big enough, you can even shoot them with no sizing. But this trick does have a liability: For some reason, it seldom gives high precision accuracy. You won't lose a deer because of it, but you could lose a groundhog that's way over yonder. A 4 to 6 inch group at 200 yards is about what I'd expect from maximum loads, though some guns do quite a bit better, and some do worse. You'll just have to try it and see what kind of accuracy you get with your loads, and decide if it's acceptable for you. As with anything new, start low and work up gradually, changing ONLY one thing at a time. This has been gone over in some detail in previous postings. Do a search for 'COW loads" or "Cream of Wheat".

[Dannix]

But this trick does have a liability: For some reason, it seldom gives high precision accuracy.

Didn't realize that. Thanks for posting Molly!

[.357MAN]

I know this is an old thread but I joined CB so I could post my opinion.
Even in my brief experience with cast bullets I feel I have learned quite a bit thanks to CB.
I can't help feeling that lots are over thinking this, me included. According to everything that's been said:
(1) Gas check are to either: Shield the bullet in one way or another from hot gas through gas cutting (<more popular view), or to give the bullet a higher shearing strength.
(2) Leading is caused by poor bullet fit, thus vaporizing lead by gas blowby along the bullets shank and depositing lead along the barrel, then the bullet ironing it onto the barrel surface.
(3) Lube is to either: Blocks gas blowby ( <more popular view), or lube the bullet shank as to reduce friction.

I conducted some unprofessional tests to see whether my alloy was up to my standards of presser I wanted to run them at. I started with an unknown lead alloy so I got a Lee's hardness tester ( It killed me to spend the money, but I really wanted to now my alloy's BHN ) . It gave consistent reading to within +1.5- BHN. My alloy was within 1 BHN of pure lead, but I thought "what the heck it'll go down the barrel" so onto the load development part. To make sure the bullets weren't striping in the rifling I shot one out of each test lot into milk jugs ( yeah, low tech. ) then retrieved the bullet and checked the riffling engravings. Needless to say the bullets started to strip at about the fourth lot ( 38 Spl. 158 Gr. TL PB bullet with LLA, 3.4 Gr. Trail Boss, guessing around 12,500 PSI ) of the planned to be eight lots. So I got taught about sheering strength fairly fast. After that I richened the alloy with Lino type to make a BNH of 10.4 +1- , and now with same bullet and lube using 4 gr. of TB and LLA I am getting good groups at 25 yards.

So going along with others I think the gas checks are to aid in shearing strength of the cast bullet alone. Before you members that don't believe this start to hammer me, think about it. Imagine a bullet like mine pushing 13,700 PSI ( Max for TB in my setup ) the bullet would be totally striping in the rifling. Why? Because the leads max shearing strength has been exceeded, so it strips the rifling, causing leading. Now, imagine a bullet similar to mine but with a GC base with a GC on it. As the bullet is starting into the rifling it starts to strip ( keep in mind the bullet's whole shank doesn't fully strip the rifling until the bullets is completely engaged by the rifling, cause it's the rifling twisting that strips the bullet from the rifling ) from the front to the back (As the bullets soft alloy can't start the bullet twisting ) once the GC gets to the rifling the GC is crushed into the shape of the rifling. Which in doing squeezes the bullet to conform to the new shape of the GC. The GC would look more like a socket wrench hex shape ( if there is 6 rifling groves, if 8 groves then octagon shape and so on ) than a cylinder shape. Now the GC acts just like a wrench to turn the bullet ( now, since the bullet just started into the rifling the bullet is not fully striped of its groves in the rifling) keeping the last little bit of the bullet closest to the GC from striping and letting gas melt the shank, causing leading.

Now bear with me for a little longer, as I am not done.

Some members have in deep detail told about the way leading is formed in the barrel due to gas cutting.
But I think It's more than just gas cutting.
Imagine a .358" PB bullet traveling down a 6" barrel lets say at 1109 FPS with 23,900 CUP (max for HP-38 in .357 MAG.) behind it, and the barrel has a .001" restriction in the first one inch then goes back to normal diameter. Well now there is a bullet that is not making a perfect bore tight seal in the barrel at the most critical point of pressure. What would happen? Hmm... lets think about it. Maybe the gas would blow past the bullets shank? Yes. Then what? The bullet will leave leading in the barrel. Well yeah! duuuuu! Why dose it lead?
Most think the hot gases have a blow torch effect on the bullets shank eating away the alloy at an almost microscopic level and this then gets deposited onto the barrel further down. Other opinions include chatter down the barrel causing leading (the latter is not really in this thread but I have heard it on others ) and a few others that I can't remember.
From what I hear gas cutting sounds the best and most sensible. But this thread got me to brain storming some ideas. When the gas blows by the bullets shank I think more than one thing happens from blowby.
(1) All the lube in the area gets air blasted off the lead and the barrel.
(2) The lead bullets shank that's exposed to gas gets heated, no one knows how much, but it gets hot.
(3) The gun barrel exposed gets hot, like I said before, no one knows how hot.
I think the three together make the perfect environment to solder the hot lead right to the barrel from the bullet's shank.
This would explain why most leading is specific to the area of the bullet that is under size, and why it looks like smudges and not small ironed on beads / splatter (although some could look like this, when I got leading it was smudging ).

Ending with the lube subject, I think its effect is to seal off hot gasses from escaping around the bullet's shank thus welding the exposed lead to the exposed barrel surfaces. That's why so many members have good successes with high presser greases, they are always thick and gummy in wide temperature ranges, and they seal the imperfections in-between the bullet and the barrel better than other lube.

Sorry It's so long, but I couldn't hold it in, I had to put my 2 cents in.

.357MAN

[Molly]

Hi .357MAN,

>I know this is an old thread but I joined CB so I could post my opinion. Even in my brief experience with cast bullets I feel I have learned quite a bit thanks to CB.

Yeah, this is a great site where almost anyone can learn a new trick or two. For example, I had never considered the rifling making a sort of toothed gear out of the gas check, and thus assisting in bullet rotation. Could be. There's no rule that says something can't work in several different ways. But you've got to be careful not to overlook contrary data either. For example, Col. Harrison showed rather convincingly that even pure lead can't be stripped by rotational stress alone.

>(3) Lube is to either: Blocks gas blowby ( <more popular view), or lube the bullet shank as to reduce friction.

Lube may have some slight gas blocking or friction reduction effect, but that's probably not it's major function. In fact, lube isn't even necessary for decent accuracy. Air rifles give superb accuracy from pure lead at over 1000 fps without a trace of our greasy lubes. And even using our cast bullets, you can eliminate the lube (and the gas check) and get decent accuracy with CoW loads.

>so I got a Lee's hardness tester

Did you consider the pencil hardness test method? Works great, costs pennies. There's a thread on it here somewhere. Do a search.

>Needless to say the bullets started to strip at about the fourth lot ... After that I richened the alloy with Lino type to make a BNH of 10.4 +1- , and now with same bullet and lube using 4 gr. of TB and LLA I am getting good groups at 25 yards.

I've seen bullets with engraving noticeably wider than the lands, but it wasn't from rotational stripping. It was from abrasion against the sides of the lands. I HAVE seen shear stripping in a barrel with VERY shallow rifling. It was an interesting experience, because you could look down the barrel and see thin strings of alloy all over the place. The shallow rifling apparently acted a lot like a potato peeler, and left the peelings all over the bore.

>So going along with others I think the gas checks are to aid in shearing strength of the cast bullet alone. Before you members that don't believe this start to hammer me, think about it.

Let me recommend that you review Col. Harrison's research on this before you set that opinion in concrete. You can get a copy of his report from the NRA for free.

>Now the GC acts just like a wrench to turn the bullet (now, since the bullet just started into the rifling the bullet is not fully striped of its groves in the rifling) keeping the last little bit of the bullet closest to the GC from striping and letting gas melt the shank, causing leading.

Like I said, the wrench theory is interesting, but the base / shank of the cast bullet simply doesn't melt in bulk with any reasonable load. Proof of this can be picked up at any range, where it is easily seen even jacketed loads at notably higher pressures and temperatures of even jacketed loads isn't severe enough to do the job.

Now bear with me for a little longer, as I am not done.

>From what I hear gas cutting sounds the best and most sensible. But this thread got me to brain storming some ideas. When the gas blows by the bullets shank I think more than one thing happens from blowby.
(1) All the lube in the area gets air blasted off the lead and the barrel.

Nope: Cast bullet recovered and examined show plenty of lube still in the grooves after shooting.

>(2) The lead bullets shank that's exposed to gas gets heated, no one knows how much, but it gets hot.
(3) The gun barrel exposed gets hot, like I said before, no one knows how hot.
I think the three together make the perfect environment to solder the hot lead right to the barrel from the bullet's shank.
This would explain why most leading is specific to the area of the bullet that is under size, and why it looks like smudges and not small ironed on beads / splatter (although some could look like this, when I got leading it was smudging).

>Ending with the lube subject, I think its effect is to seal off hot gasses from escaping around the bullet's shank thus welding the exposed lead to the exposed barrel surfaces. That's why so many members have good successes with high presser greases, they are always thick and gummy in wide temperature ranges, and they seal the imperfections in-between the bullet and the barrel better than other lube.

The microscopic droplets etched off by gas blowby are melted as part of the etching process. They don't have much opportunity to cool as they are carried into the bore by the hot gas, so when the bullet slams past, they are still soft (if not still actually molten) and easily smeared by passage of the bullet.

And while lube probably does have some sealing function, I believe the evidence shows that their primary function is to contaminate the bore and prevent soldering by the etched alloy droplets. Think of trying to solder dirty steel or copper and you will understand. Then try this: Load up a bore mop with lube and scrub the bore before each to be sure every surface is coated. You will find that leading can and will still occur with hot loads, despite all the sealing function you can add to the bore. The reason is this: The etching action of blowby doesn't have any problem melting lube and spraying it into the bore ahead of the bullet, just as it does the lead. This molten lube is quite low in viscosity, and wets out the bore (IE contaminates it) and prevents leading because its surface tension is lower than the surface tension of the melted lead etchings. The molten lube thus preferentially wets the steel, and prevents wetting by the molten lead alloy. But with increasingly more powerful loads, the higher temperature of the gases result in higher temperature and lower surface tension of the etchings. Higher temperatures also promote decomposition of the lube, which is thus less able to wet the steel bore. Remember, the flame temperatures exceed the melting point of steel. Sooner or later, the increasing wetting ability of the hotter allow etching overcomes the decreasing ability of the lube to wet and contaminate the bore. The result is leading.

>Sorry It's so long, but I couldn't hold it in, I had to put my 2 cents in.

Put in ten cents worth. It's always good to hear from someone who is interested enough to actually run tests and think about what the results could mean. You're welcome here indeed.

[.357MAN]

Hi Molly,

I am sorry to say I have never read Col. Harrison book, but I hope to acquire a copy in the future. I am certain it will be enlightening to say the least.
Yeah, the more I think about it, the more it makes sense the rifling would not fully strip the bullet in normal rifling. But it would still make wider engraving such as you experienced, which in theory would cause leading by gas blowby.

I completely agree with the CoW, it makes perfect sense( although I did not mention it in my first post ). From what I can grasp of CoW is that it fills the role of the lube ( makes sure gas doesn't blowby the shank / side ). Using only CoW also suggest that the lube has little if any lubricating effect on the two surfaces. That's why I say lube is only to seal the gas from leaking around the bullet.

I agree with you that it's abrasion against the sides of the lands. I think I used "striping" wrongly to convey what I meant. But it is the shearing strength of the alloy that resists the abrasion / cutting and turn's the bullet. That's why GC help so much with softer alloys, they add shearing strength to the bullet.

"Shank" Again a bad usage by me, I meant the sides of the bullet. When there is not a tight seal the shank / side are exposed to the hot rushing gases, thus blowing lube away and melting the shank / side and smudging lead on that nice clean bare spot in the barrel.

Yes! bullets have still have lube on them even after going through 6 1Gal. jugs of water.
I am saying at the point of the gas leek ( not the entire bullet surface ) it is folly to say that the lead is melted and blasted away and not the lube. Most lubes flame point is at 250 F. +100- degrees, yet lead melts at 600 F. to 700 F. .

Yes, there could be micro droplets from blowby but its insignificant. But how do you explain after one to three shots that there is a large smear in a specific area? Another thought is, if indeed lead was melted by gas ( <I agree with this) ( not this >) then blown forward of the bullet, then the bullet ironing it on, causing leading. If so, then the lube forward of the bullet would protect the barrel, and the bullet would then scrape the lead off ( according to you, this is one of the main effects of lube ) and we would never have leading in light loads even with soft or under sized bullets.

I have soldered, and I have welded, and I know what you mean. But then why is there leading with lube more than CoW? Because CoW is thicker and so a better sealer than lube, thus no gas could pass the bullet even with a bullet that has small openings which would otherwise let gas leak destroying lube and soldering lead to the barrel.

As far as swabbing the bore with lube, of course it wouldn't help. Lube has to be behind the bullet so the pressure would force the lube into the imperfections, thus making a tight seal.

If it's true that lubes primary function is to contaminate the bore and prevent soldering by the etched alloy droplets, then why would I get leading with extremely low pressure loads that would not exceed the lubes abilities to wet the barrel?
Because it's not wetting the barrel, it is sealing the imperfection to stop gas from melting the shank / side to the barrel. That is why I got leading. The imperfections on the bullets were to large for the lube to seal, thus gas blowby blew the lube out of the area and heated the two surfaces which then smeared lead, leaving leading.

I hope I have explained in a better manner than my first post. Thank you for welcoming me, and it is appreciated greatly.

P.S. sorry I can't quote you, as I am new and not a computer wiz.

.357 MAN

[johnnybar]

"2. That it prevents the super-hot sand-blasting effect of the powder gasses from melting the lead base of the bullet, which in turn leaves lead in the bore..."

The hot gasses are not in contact with the base long enough to melt it.

I did an experiment not long ago with a cutting torch(1800+°), I applied this heat to the base of a 30 cal WW boolet with someone using a stop watch, it took aprox. 2 seconds to show signs of melting on the base.

Did your torch also provide a pressure differential of 10,000 to 30,000 psi across the area being torched? I'm being a bit sarcastic...obviously it did not. Pressure is a major force to bring under control in reloading naked slugs. Look at what it and temp can do to a little graphite....diamonds.

[johnnybar]

An example of extreme pressure to keep the think tank going here:
Here we have pressure acting on lead and steel plate. Insert enough pressure into the equation and lead can literally splash steel like soft butter. In our leading challenge, we have high pressure gasses vs. lead, GC and lube. Lots to think about....

[fcvan]

For decades, I only used gas checks when I launched beyond 1200 fps. I would buy 1000 checks and they lasted a long, long time. After using the last of my checks purchased at 10 bucks a thou I was shocked to see what they want for checks these days! Heck, I used to use checks as under and over shot wads for snakeshot rounds which used them twice as fast. Then I bought some CheckMaker dies, some regular and some plain based CheckMakers. I'm putting gas checks on boolits that are under 1200fps and don't really need them. Checks are like spinners on a 67 Chevy. The tires don't roll any better but they look cool

Ok, back to seriousness. Plain based checks on non magnum loads can be a crutch for folks who haven't found the fit, lube, and powder combo and are still experiencing leading. I have a buddy who prefers to shoot jackets because he's worried about leading and would rather not risk it. He will shoot gas checked boolits so it gives him piece of mind. I like reloading which for me now includes making my own checks in addition to pouring my own. My next endeavor will be making my own black powder! Frank

[Ray1946]

Hello all! Yes, I have sat here and read all 6 pages of this very informative thread. WOW! I think my brains ready to EXPLODE! Since everyone has had there say, I think it might be a good idea for me to include mine.

I started reloading cast bullets in a Mosin-Nagant in 1974. Back then I was getting Norma brass for $.20 a pop. I bought some Lee molds and began casting some pretty nice boolits. These came out of the mold .3118 and so I shot them unsized, but seated a Hornady gas-check. The groove in that rifle was a solid .312, so I was slightly on the low side of the groove diameter. Some of the beginning loads back then were more than maximum by today's standards. I got great results if I didn';t go over about 1400-1500 fps with most of the fast rifle powders;4227,2400 RL7. I fooled with this for 3 or 4 years while reloading for a .222 with jacketed bullets.

Slide on down to 1981 and becoming interested in 100yd. highpower. In my area there were not a lot of really close 2,3, and 600 yd. ranges, so the big deal at that time was shooting reduced to 100 yds. Looking around at something that might be competive and AFFORDABLE, I opted for a 96/11 Schmidt-Rubin. This is one incredable rifle! Mine, along with every other one I have ever seen has a near perfect barrel that is a whopping 30.7 inches long; right at 36in. between the front and rear sights. I epoxy-bedded that action, bought a Williams micrometer sight for the rear and a Lyman globe for the front and we are off to the races. My most accurate boolit was the Lee180309 at about 1650-1675fps, the lube was Alox 50-50 and I managed several .3 and under 50yd groups from a rest. I later bought an NEI 195311. This is a really beautiful bullet and would perform great using RL7 at about 1750 fps.

The whole thing about what I learned was that in High-power, you have to shoot from 50 to 90 rnds WITHOUT cleaning,so if something breaks down, you are going to be all over the target for a whole lot of rounds. The groove in the old Swiss was .3094, so I sized my boolits at .3105 and this seemed to work very well. At the time I was using straight Linotype and although this made a wonderfully HARD boolit, the boolit also had a lower melting point than say No.2 alloy. I think the melting point of the alloy has a significant effect on its plasticity as it absorbs impact and heat for a micro-second of time. With Linotype, once a certain pressure/TEMPERATURE threshold is reached; you get leading, I think mainly because of the lower melting point of the alloy.

Another factor I think adds to good accuracy and consistency is the lubricant. I think lube is not just a lubricant, but also a COOLANT. I shot several cast bullets lubed with a moly dry-lube and gas-checks, but NO lube in the grooves and they leaded terribly. The same dry-lubed boolits with lube in the grooves performed flawlessly...........................

[morton45]

It's been very informative to read the various theories & opinions on the subject. I suppose that through my experience lead hardness definitely had a role to play. For example soft lead Hollow base wadcutters are great in my 6" 586 S&W at 900fps, however in my 18" Browning M92 I can't load them much higher or I will get leading or worse they will separate. I believe the seal between projectile & bore is very good at this lower velocity & pressure. However when I try for more speed I am in effect deforming the projectile due to higher pressure & more powerful ignition.
I believe after a point the gasses burning the bottom corners or edges of the projectile & each subsequent shot will smear the previous deposit along the bore.
The faster the projectile the harder the alloy until you need a gas check. One shooter suggested they had shot thousands of US Military issue jacketed rounds where the base of soft lead was exposed & no leading had occured in this instance. Well I believe that's because the jacket wraps around the bases edges or corners. This is the weak point. I have found when melting anything be it wax, lead or even cutting with the Oxy torch, it's much easier on a corner to start the melt than in the centre, although that can be done with greater heat.
Lube is, as Ray above suggests both lubricant & Coolant. The same principles are at work in your average internal combustion engine. The oil lubricates the moving parts but the oil also plays a critical role in carrying the heat across for the cooling system to radiate externally.

Just my thoughts, Cast bullets are fun & challenging. I'm sure gonna research these COW loads. Thanks to all.

[billurbank]

Some years ago ML McPhearson wrote an article on building a boolit trap that does not deform the boolit. He used a sheet of plywood cut in one foot by eight foot strips to mke a box eight foot long. Fill the box with fine saw dust from the saw mill and saturate the sawdust with hydraulic oil. I built this and can tell you that the results are better than hoped for.
Because I can see uncorrupted proof of the changes I make in my experiments, I believe it keeps me on a narrower path. I have satisfied myself about lubes, bore alignment, boolit/rifling, and terminal performance as well.
I plan to compare the three foot water column trap to the sawdust.
The point is, speculation can be moved forward

[LeSellers]

As a newby, I have no opinion, much less any helpful information on gas checks. I do have a question, however.

Whatever function a gas check has, is it duplicated when a bullet (factory) or boolit (hand cast) is copper plated?

If we assume that a gas check keeps barrel leading to a minimum, would a very thin layer of copper plated onto a similar boolit keep the barrel fouling to the same kind of level?

Le

[psychicrhino]

Questions answered I didnt know I had

[garand41]

About the black powder. If you would shoot a muzzle loader at nite, you will find that a lot of the powder is expelled, on fire, from the muzzle. Black powder does burn down the barrel, it does not explode!

[giericd]

i know this is an old thread, but some of us are new to the site and just comming across it- so by cointiuing to add to it might help some of us new guys. im new to casting and had some thoughts/questions about gas checks. i've played around with heating a lot of metals/aloys. I've done a bit of blacksmithing so i understand hardness of metals vs. hardness when heated. there is no way you can take a tortch to a cast bullet and see how long it takes to melt and compare that to what happens to the metal under the same temp under thousands of PSI! this much i know. my question is has anyone experienced and damage to their bore (sctatches/scrapes/groves) due to the gas check rubbing the walls? Also a safety question- if it seperates from the bullet after leaving the barrel the bullet heads in the direction you aimed but what happens to the gas check? sorta like throwing a CD, that sucker could exit the barrel, take a 90 degree turn to the right, then back left who knows but it could end up hundreds of yards off course to the right or left where you didn't intend for a projectile to go. As far as seal goes i highly doubt it, at .005" how much seal are you going to really get. any seal you would get would be due to the diameter of the bullet like .224 in a .223 rifle or a .401 bullet in a .40S&W ya might get a little more seal with cast due to it being a softer metal and the midsection of the bullet expanding (flexing) slightly when the round is fired. as the pressure of the round going off hits the base of the bullet it starts moving forward before the fromt of the bullet by a fraction of a second, for that breif time the lead in the base that moved forward would have made the mid section expand slightly creating more of a seal. so my theory would have to come down to the gaschecks only purpose is to prevent the base of the bullet from deforming/melting under heat and pressure causing leading and destroying accuracy, and since it is made of a more heat resistant material than lead and equally spreads out the pressure the base of the bullet is subjected to it allows for more powerful loads and to reach faster bullet speads than lead alone would allow. just my thoughts. what do you guys think?

[blackangus]

what filler for a 30-30, if my bullets are not rebated?

[kolob10]

Great compendium of the wealth of knowledge and experience on this site. Thanks! I would only add that many years ago a was at an insurance underwriter test facility and witnessed a demonstration explosion of some corn meal dust. The chamber in which the explosion originated was a bunker dug into the side of the hill with one side exposed but covered with a "plastic curtain" that appeared to be an industrial shower curtain. The bunker was designed to direct the force of the blast (path of least resistance) toward those who watched the blast through the plastic curtain. The onlookers were placed behind a transparent blast shield 100 yards distant.

After the blast, I was curious as to why the "plastic curtain" was placed in front of the bunker. The chief Engineer explained that it was used to absorb the heat energy coming from the blast to make the demonstration more viewer friendly and prevent grass fires. Indeed I felt the blast but very little heat energy reached the viewers.

After I returned home I wondered if I could do the same thing for the PB cast bullets I was shooting in my 357 mag to reduce leading. I knew nothing of matching the sized bullet to the groove diameter at the time as I purchased all my cast bullets locally.

I did some research and decided to use styrofoam. I sharpened a 357 case neck (used as a cutter) and proceeded to cut some disk out of a styrofoam egg carton. I placed the syrofoam disc on the top of each powder charge prior to seating the cast boolit. It worked pretty well to my surprise.

Later I learned that NASA placed an "ablator" or head absorbing shield on spacecraft to reduce heat damage. I'm a gas check guy myself on everything except my 44 Special loads. Old thread I know but great info.