Gas Check Thickness

[.30/30 Guy]

My first experience making gas checks was with the early version of Freechex, the kind that you This summer I have sent in the neighborhood of a thousand .30 caliber cast boolits down range fitted with homemade gas checks. I have just started making gas checks for pistol.
would whack with a hammer or club. I no longer own such items.

My experience this year has been with PatMarlin’s Checkmaker in .30 and .35 calibers. I have been very pleased with the gas checks that I have made with the Checkmaker.

I have developed several theories about home make gas checks.

Theory #1
With bore riding boolits the gas check must act as a driving or guiding band. The fitted gas check should be the same diameter as the boolit band above it. The gas check should be swaged onto the boolit shank during the sizing operation.

The minimum thickness of the gas check can be calculated as follows:
T = minimum thickness or gas check material
SD = diameter of sized boolit
GD = diameter of gas check shank
T > (SD – GD)/2
( “>” means greater than )

Theory #2
With full bore boolits such as pistol or Loverin style boolits (non bore riding) the gas check needs only to be secured to the boolit . The bearing length of the boolit overwhelms the bearing effect of the gas check. I cannot prove that it makes any difference, but I like the gas check to arrive at the target still attached to the boolit. My experience with the .357 Magnum has been that the thickness of the gas check has minimal effect on the boolit accuracy. My testing to this point has been that 0.008” lith plate checks performed a little better than 0.014” aluminum.

OK guys have at it! I am sure that I will get plenty of comments and feedback about my theories.

[Wally]

In the .357 Magnum seems .008" & .014" thick GC makes no difference. I would guess that the thickness is reduced, on the sides of the thicker stock, when sized anyway. I use the thicker stock in mine.

I don't have any bore riding bullets... I have used home made GCs in reverse on PB bullets and they have worked superbly--

1) Lee 140 SWC .38 Caliber in a .357 Magnum

2) Lyman 311410 .30 Caliber 130 RN in a .30-06 Springfield

Both loads were "on par" with the same accuracy that I had been getting with regular GC bullets. Thye are a tad tricky to seat...but it gets easier w/practice.

[Von Gruff]

Theory #1
With bore riding boolits the gas check must act as a driving or guiding band. The fitted gas check should be the same diameter as the boolit band above it. The gas check should be swaged onto the boolit shank during the sizing operation.

The minimum thickness of the gas check can be calculated as follows:
T = minimum thickness or gas check material
SD = diameter of sized boolit
GD = diameter of gas check shank
T > (SD – GD)/2
( “>” means greater than )

.

Did you come to any conclusion as the the value of >. That is , how much 'greater than' are you talking about. When does swaging the GC onto the shank become delitorious rather than advantagous.

Pat is making me a checkmaker for my 7mm boolit that has a .2545shank and is sized to .2855 for a .031 difference The intention is to use some .021 copper which will give a swaging of .011.
This for a 2400fps load I have

A double aluminium check of a .006 inside a .012 for a .018 and a .006 swaging works well up to about 1900fps but the AL dosent seem to like the higher velocity.

While my boolit is a bore riding design the nose section is .001 larger than the bore dia so I nose size enough to seat to proper length.

Von Gruff.

[.30/30 Guy]

I have not tried any material thicker than 0.014". I think that the limiting thickness is the maximum thickness that you can form with the die and not stick to the forming punch.

[JeffinNZ]

I have found overwhelmingly that it is critical to completely fill the void between shank and throat/groove so your formula is correct though I would be inclined to say:

T =/> (SD – GD)/2

> is good as it generates a little extrusion of the material and makes for good crisp checks.