GAS CHECK MATERIAL – IT AIN’T ALL THE SAME.

I have been making my own gas checks for many years now.  My experience is 100% only with Freechex tools from Charlie Darnall of California from whom I bought my very first device, a Freechex I, in 2009.  Since this time Charlie and I have become great friends and we have worked together on many refinements to his tools with him making and theorising and me shooting, testing and ‘monkeying’ (Charlie’s word not mine) the tools to get them to do what I want.  Very early on I had to drag young Charles kicking and screaming to convince him that a standard .30cal pattern tool was not suitable for making checks for oversized .303 British bullets.  Along the way I have used many different types of materials for making my gas checks most of which has been aluminum (aluminium for us Colonials) but soon discovered that there is a vast and varied array of aluminum alloy available and it is far from equal, certainly in terms of making and successfully shooting gas checked bullets.

From day one it became apparent that my .303 British bullets needed 0.016 inch thick material to make checks fit for the job.  To determine the thickness required for a gas check one simply subtracts the bullet gas check shank diameter from the final bullet sizing diameter and halves it.  

X = (SD-ShD)/2 where SD is sizing diameter, ShD is shank diameter and X is the material gauge.

When sizing to 0.316 on a shank of 0.284 the material had to be AT LEAST 0.016 inch and for love nor money I could not find any.  Best I come up with was a 2 ply check with the outer layer being 0.012 lithography plate and an inner layer of 0.004 soda can wall.  This made fantastic checks that shot amazingly well but were a bit of a hassle to make.  One day it become apparent the wall thickness of a deodorant can was the perfect gauge – VIOLA.  Test loads were assembled and shot with hideous results.  The material was very soft aluminum alloy and clearly too soft for the job at hand.  It seemed that aluminum alloy had to be in the 3XXX series to make decent checks and this was deduced from research into the lithography plate.  When dealing with aluminum alloy the lower the number the more pure the material, the softer the metal is.

An English gent by the name of Ian Yerlett came on the scene via the Cast Boolit Forum (Yonky) and began supplying cranks like myself with suitable material from his slitting mill.  I ordered some coils at 0.016 inch from Ian in both 3XXX series aluminum and copper.  Both shoot very well BUT the copper shoots BETTER when velocities begin to exceed 1600fps in both my .303’s and .30/30 Win.  Case in point:

I have a beautiful CBE 309 162 mould for my Husqvarna No35 .30/30 WCF rifle.  Cast in 40-1 alloy the bullets are just shy of 170gr and .301/.311 diameter.  Over 22gr of Benchmark (BM2 from ADI) the load produces 1750fps and will group wonderfully well at 50m BUT only with copper or brass checks.  Even Ian’s lithography plate spec coil will not shoot on this load within a bull’s roar of the copper and brass checks.  Initial tests with this bullet were conducted with aluminum checks and were very inconsistent.  Out of desperation I prepared some Hornady commercial checks in a duplicate load and hey presto, instant accurary.  See below for comparison, Cu left, Al right:

3030 Cu v Al

It transpired that (IMHO) the 40/1 alloy is sufficiently soft that at the velocity and/or pressure the load produces, the bullet requires a gas check with a significantly greater degree of grip on the rifling that can be afforded by aluminum alloy.  Tests with half hard brass produced indentical results.

I published these results on a couple of shooting forums and James Sage from Sage’s Outdoors contacted me asking if I would like to test his commercial aluminum gas checks for comparison.  James uses Charlie’s tools exclusively in his commercial operation.  Via USPS I received two packets of .303 spec checks; one in 0.015 ‘lithography plate material’ and the other 0.0155 ‘Amerimax flashing’.  James suggested the ‘litho’ checks would produced results akin to Cu.  Today I finally got out to test both.

Using the same load of a 40/1 CBE 309 162 over 22gr BM2 at 50m I shot both varieties and checks.  The sun was playing games on me this cold, cold winter’s day and unforunately I split the group on the first target when clouds came into play but the reader will get a pretty good indication of how the ‘litho’ material checks went (note the first shot after changing material I have found to be stray).  The softer flashing grade material did as was expected – not so well.

“Lithography grade 0.015”

DSCN4691

Flashing material:

DSCN4692

Clearly there is a difference between the nominal ‘litho’ material supplied by Ian and James and without know the alloy number I can’t comment further other than to say Jame’s material worked.  I have commented to other shooters in the past that I would need to take a full month off of work to spend experimenting to get to the bottom of the quirks of making and shooting your own gas checks such are the possible combinations to try.  To this day I still can’t be sure whether the limit of material is a factor of pressure or velocity or both.  I suspect pressure however.  Certainly what I can be sure of is it is not at all safe to assume that because your gas check shoots MOA with a low pressure load at 1400fps (my .303 Brit target load) that similar accuracy can be expected at the other end of the velocity/pressure spectrum – it can’t. 

Once I have this completely worked out you’ll all be the second to know. 

Keep your powder dry.  Jeff.

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About JeffinNZ

Dedicated cast bullet shooter, runner, fisherman and father for two beautiful girls.
This entry was posted in General, Handloading. Bookmark the permalink.

One Response to GAS CHECK MATERIAL – IT AIN’T ALL THE SAME.

  1. Dick Boothroyd says:

    If I can’t get material of ideal thickness I’ll take the next thinnest and bore out the mould to suit. If its a custom mould (usually the best way to go) have the shank cut to suit the material. And, of course, have the check making tool made to suit.

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