Seating Gas Checks

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  • Last Post 29 September 2021
cove posted this 05 September 2021

I have spent a lot of time fiddling around to make sure gas checks of my 30 cal bullets were seated squarely. Then, the other day it dawned on me that upon firing there is more than 20,000 C.U.P of pressure on the GC upon firing.  I would imagine this sort of pressure would firmly seat the check and squareness would be determined by the squreness of the base of the bullet. Any comments?

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Tom G posted this 05 September 2021

My experience has shown that is doesn't work that way.  The reason being is the checks are crimped onto the bullet base and if they are crimped crooked, they will cause fliers. You  have to seat them straight and then crimp them for them to be accurate.  I never shot any that just slip on and don't crimp so I can't comment on what would happen if the check is not tight when shot. 

A few years ago, my brother sent me some 22 cal bullets he had made to try in my .223 AR rifle.  They shot into a nice core group but has several fliers. When I looked at the ones I hadn't shot yet I set them up on a flat surface and it was easy to see the ones that were crooked. These were not seated first and then sized. He learned a lesson not to skip seating the checks first before sizing in a lube sizer. If they were seated straight, they shot with quite good accuracy. 

Tom

 

Tom

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4and1 posted this 05 September 2021

They have to be square and flat. I made a die that holds the bullet and I seat the GCs using an arbor press, giving them a good  hit. Then my sizer has a stop that is faced square, and the bullets are crimped going into the sizer base first and they seat against the stop.

 

I don't like the Hornady checks that have a cup in the bottom, that has to be mashed out flat, IMO.

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MP1886 posted this 05 September 2021

A lot of suppositions here.  Simply put gas checks are to be fully seated and when they are, unless the base of the bullet isn't square, they should be square.  Making tools and going anal to square up a gas check or even sanding/filing them to get the same thickness is a waste of time.  I chuckled about crimp type gas checks not fully seating with the high gas pressure behind if they weren't fully seated is crazy.  Think about bullets with a crimp groove or jacketed with a cannelure. Doesn't the gas pressure push they out of the case???? One can fully seat gas check with their Lyman/RCBS luber/sizers using the gas check seater.  There's ways around this too without having to buy the seater if you're use that type of equipment. You know boys unless you have a match grade barrel and rifle it's a waste of time using all the benchrest techniques like weighing your cases (really you should be checking the internal volume rather then weighing them) uniforming the primer flash holes, etc is a waste of time. Probably one of the most important things for cast accuracy is casting GOOD bullet with a minimum weight variance of say like .1 grain.  Okay pile on me....LOL

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45 2.1 posted this 05 September 2021

Probably one of the most important things for cast accuracy is casting GOOD bullet with a minimum weight variance of say like .1 grain. 

He has a very good point there. Those bullets are much better than bullets sorted out of a much larger cast lot. The problem is using a SC mold and a dipper to learn how it's done.

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Shopdog posted this 05 September 2021

Yup,the reason to weigh bullets isn't about segregation,it's the check on your casting setup.

I use a small jewelry C frame press to seat GC's. Here again,it isn't solely about say,getting them seated deadnuts square(although this is highly important)... in this process I get a visual on how the fit is. Tough to describe,stupid easy to see.

One way to "set yourself up" for failure.... and the point here is another visual. Try nose sizing dry vs lubed on a long'ish bore rider. Further,do some checking on the taper during this op. What you're going to see is how lead differs so much from JB in it's "righting moment". Once lead starts out,"wrong".... it gets worse. You should be able to very easily distinguish. So,be careful dismissing a process as unnecessary because,you can't shoot the difference. It isn't always about,just that.

Good luck with your project.

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John Alexander posted this 05 September 2021

Excellent question Cove,

There are lots of opinions and assumptions in the above posts. Why not, that's the way we usually discuss cast bullet issues and there is no harm in batting around our ideas and opinions. But opinions, no matter how confidently stated aren't facts.

BUT -- Do we know of any decently designed experiments complete with controls, enough shots fired to reasonably confident of the results to support any of the procedures above.  Yes, I know plenty of anecdotes, plenty of "its just logical", and  plenty of "a really good shooter does it".  But without experimental back up we don't really know do we?

 High precision is just a waste of time unless it produces a measurable and repeatable improvement not just "it seemed to help" or "the next couple of groups were better".  

If anybody knows of any decent experiments to back up any of these opinions please let us all know.  That's the only way we will make any progress.

I don't mean to be a party pooper, but why in the hell don't we experiment to confirm or disconfirm the procedures we think up instead of just thinking up more nits to pick? It would make much more interesting discussion and we might learn something new.

John

 

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OU812 posted this 06 September 2021

Gas checks that fall off during flight will cause bad accuracy. My croni is peppered with dents. Usually the gas check will seperate from bullet as soon as bullet exits muzzle. How do I know this? I once experimented sanding all gas check skirts equal length. The cure was to never again block sand gas check skirt to equal length. This removed the sharp edge from gas check after being punched out and formed.

A good square and tight fitting gas check is the bomb...along with great fit and alignment.

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OU812 posted this 06 September 2021

I do not believe in annealing. Why anneal? Wouldn't that just make them softer and weaker. Less clamping force when exiting muzzle.

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Ross Smith posted this 06 September 2021

My experience with gas checks is that if the cast bullet base is too big or has flashing, the checks won't always seat straight. I even talked to the people at Gator gas check. I'm convinced the problem is not the gas check. After trying many forms of seating, I use my rcbs lubsizer to seat checks in the sizer die. Any that aren't straight go back in the pot.  Some molds cast bullet bases too big . Also alloy changes base diameter. I retire problem molds.

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Brodie posted this 06 September 2021

I agree with Ross, if the base is not square or there is anything that is not uniform about it the check will be off and tilted to one side. 

I seat my gas checks when I size the bullets normally in push-through sizers, I think that the force needed to size the bullet down is all that is necessary to force the bullet base firmly into the bottom of the gas check.  If the base is crooked or has a fin on it the gas check will not be square and if set on a hard surface the bullet will either fall over or sit with a decided tilt.  It ain't rocket science,it is casting better bullets.

B.E.Brickey

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4and1 posted this 06 September 2021

I am moving away from base pour molds. It just doesn't make sense. Anyway, I made a very simple tool to debur any fins that may be on the base before seating them in a GC. Takes about 5 seconds and there is a smooth edge to go into the check.

You can't see or measure a check that isn't square good enough to make it fly under the rpm's and fps.

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John Carlson posted this 07 September 2021

NOE makes a tool similar to a case neck chamfering tool which does a nice job of removing any fins.  I've also gone to seating checks in a push through sizing die.

John Carlson. CBA Director of Military Competition.

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Wm Cook posted this 07 September 2021

I have two molds I use for .312-.313. Gas checks spin on both at times. On one the bullet shank is small and the crimp can’t grip the base well enough to hold it tight. On the other the base is too large and the check deforms in my sizer. Some of these spin. Some are crooked. On the latter I got by with a pan head screw to flare the checks before crimping. Not ideal but I got by.

So I guess bases can come out too small or too large and gas check will effect accuracy to the point they could fall off in flight. I have a check maker coming from Pat Marlin and I hope with different thicknesses of material I can get a good firm grip on both.

I’v often seen some of the seasoned contributors to CBA talk about recovery of projectiles. Sorting out fliers verses non fliers is next to impossible after a group is shot (as in Tom’s reference to his .223). However shooting a couple of five 5 shot groups with accredited bench practices in a firearm that has a proven accuracy history with correct and incorrect check seating is doable.

The big caveat as MP suggested, is that gas checks play a part in accuracy but making 25 bullets that can shoot 5 individual groups that aggregate <1” with="" cast="" bullets="" creates="" a="" long="" list="" of="" things="" and="" each="" of="" those="" variables="" has="" to="" be="" done="" the="" same="" way="" every="" time.="" bill.="" with="" cast="" bullets="" creates="" a="" long="" list="" of="" things="" and="" each="" of="" those="" variables="" has="" to="" be="" done="" the="" same="" way="" every="" time.="">

A “Measured Response” is as effective as tongue lashing a stuck door.

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Tom Acheson posted this 07 September 2021

Bumping bullets hasn't been duscussed here. But the force applied to the bullet as it is bumped will complete the check seating effort AND the bullet base will be very flat and have a sharp corner on the perimeter of the check.

Bumping also uniforms the exterior profile of the bullet which influences the fit in the chamber. I found the seating depth of a bumped bullet vs. an unaltered bullet (in my Savage .308) varied between the two bullets. And the accuracy was much better with the bumped bullets.

Another step but....

Tom

 

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Shopdog posted this 08 September 2021

Interesting you brought.up bumping Tom;

Dig out a 1990 R700VS,bone stock 308 from a safe yesterday. Was shooting 175's and doing well. Wanted to try some Lee 150's.

This particular batch of 150's cast a cpl "tenths" large on the bore ride,@ .3083-5 so was up early this am. turning up a tapered nose die to be used in the little C frame press. All went smoothly on the die. One of the first tests on its use was to go well past what the original design/depth measure was aimed at.

Now to the good part;

At that level of pressure exerted on the naked base.... it got bumped up in diameter. Didn't need to measure it because,#1 wasn't planning on going that deep,and don't with the nose size. #2,pretty durn obvious it "grew" because of how I pre seat checks(see my post above). On the as cast base,how snuggly the GC goes on is,like weighing,a quality check on the bullet.... IOW's,if the check isn't going on with the same discernable effort,that bullet has issues.

The point of the post is for folks that for whatever reason,their GC's are "loose".... you can in fact bump the bases before install. And like Tom said... after as well. Which I also tried,at the design limit. Simply put the check on before hamfisting the poor nose way to aggressively. The difference is slight,but noticeable. You just need to try it both ways and decide which is better,before or after on that particular setup. I'm NOT bumping the base on this one,for reasons stated(l like the as cast fit)..... just thought it interesting.

Shot this afternoon and would be called a liar if I posted the group size on the first 10 out of the gate. Pretty big grin is an understatement.

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Shopdog posted this 08 September 2021

Can't edit,above nose as cast with this batch is .3003-5

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John Carlson posted this 08 September 2021

NOE also makes a gas check expander tool.  Works quite well but rather a slow process and does add another step.

John Carlson. CBA Director of Military Competition.

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Shuz posted this 08 September 2021

Who makes a tool to remove fins on the base of cast boolits?

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John Carlson posted this 09 September 2021

NOE makes a tool to trim the fins off the base of the bullets.  Similar to a case neck chamfering tool.

John Carlson. CBA Director of Military Competition.

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cove posted this 09 September 2021

Well the above comments are certainly interesting and as per usual, there is not a definitive answer.  In answer to some of the comments, I anneal the Hornady GCs and the reason for that is so they do not spring back after passing through sizing die. The mold is a custom Accurate mold and there is not much resistance as the bullet passes through the sizing die, the resistance is felt when the GC is crimped into place. So, my concern is that there is very little resistance to keep the GC firmly pressed against the bullet base as it enters the push through Lee sizing die.  I have over 6,000 rounds through the barrel and have never encountered a case where the GC came off in flight (lots of shots using a chronograph). The alloy is on the soft side 20-1, so I was hoping the 20,000+ CUP pressure would force the GC firmly on the bullet base upon firing.  Accuracy is good, so I am getting the the GC on square, or the GC is squared up on firing.  As John Alexander pointed out, only a controlled series of tests with GCs seated unsquare and recovered without being deformed can answer this conundrum.

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Tom Acheson posted this 09 September 2021

Shuz,

This tool is from NOE. Cannot access their website so I can’t recall what they promote it as. But it does a good job of trimming the ocassional fins at the base of a bullet.

Tom

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JeffinNZ posted this 09 September 2021

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Shuz posted this 10 September 2021

Thanks for the info fellas. I have to get one of those tools!

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OU812 posted this 11 September 2021

I've never cast with a mould (i have lots of moulds) that produced these so called fins on base of bullet. I have pressure cast and still no sign of fins on base. Are your sprue plates tight or flat enough to prevent fins? Must be a really worn out mould?

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RicinYakima posted this 11 September 2021

Some of my moulds from the 1890's were used as pliers and make fins. Even with my best attempts to correct them, they have little fins. 

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John Alexander posted this 12 September 2021

Fins on the base are a minor pain but fins on the noses should be left on and shot as they swipe the air away so a crosswind can't push on the bullet and cause wind drift.  They also make it uncomfortable for the evil fairies that sometimes ride the bullet and steer it out of the group.  Just a helpful hint.

I have been shooting in our CBA national match all day in a switching wind with finless bullets and may be a little punchy.

John

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Premod70 posted this 13 September 2021

Great read, thanks to all for sharing knowledge.

Forrest Gump is my smarter brother.

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Ross Smith posted this 14 September 2021

Hope I'm not pirating the thread, but........ Does the NOE tool work both clockwise and counter-clockwise?

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Brodie posted this 14 September 2021

My fairies are smaller than that, John, and they can ride quite happily even in the grease grooves.

 

B.E.Brickey

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sergeant69 posted this 28 September 2021

well............i disagree. at least heres what happened to me. i bought the NOE "finner tool" based on this thread. price is right. but on my 173 gr 30 cal cast, i get fins from nose to tail. it varies on severity. still working it out why. does same thing whether i used it handle held or sent to magma and converted  for a mastercaster. lyman mold. the NOE tool does put a nice rounded edge on the base, but the fins extend upward so the GCs still won't go on all the way, as in seat squarely before sizing. so i'm back to square one. the NOE tool does NOT remove finning, it simply rounds off the base. if i'm wrong, tell me. i'd love for it to work.

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pisco posted this 29 September 2021

I made a punch to flare 30 cal g/c and it worked but have gone to using 8mm g/c on my 303 castbullets and they seem to crimp better

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