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Got a bit of a brain fart going on. If I have a bullet mould that throws a bullet designed for gas checks, do gas checks have to be used to obtain any level of performance or can they also be loaded as with any other cast bullet, with all other things being equal?

Depends on the caliber. You dont have to fit gas checks.
But if they are designed to have gas checks fitted that would suggest that the sensible thing to do is fit them.
I am sure Adam(Dromia) will explain it all to you.
I dont normally use gas checks for pistol calibers but I fit gas check to all my full bore rifle cast boolit rounds.
The gas check absorbs the initial ignition of the powder if it was not there the cast boolit would start to melt has it traveled in to the rifling causing leading.
If you need gas checks contact Adam he sells them for a very reasonable price.

Cheers Alpha, I should have stated at the begining. It's a .25-20. 90gn flat point, going to use light loads around the 1200fps mark. Unlikely this one will go out further than 50yds....

Again, hoping Adam or Roger pipe up.

If needed, can gas checks be pressed into place without the use of a press?

If you are using cast boolits are you buying them or casting them your self.

If you are buying them get them from Dodgyrog he will know wether or not you need them gas checked.
If you are going to cast them your self and need gas checks etc then Adam is your man.

Casting myself.

Picked up an almost new Ohaus mould off another forum purely by chance, about a week after I added the Winchester to the collection. Mikes at .258 and is bang on 90 grains with #2 alloy. But, designed for a gas check!

Ok I see I am not familiar with this caliber I suggest you wait for Adam to add his two pence worth.

Try them without the checks first, no point in adding an extra step and expense if you don't need to.

Theoretically you shouldn't need checks at those velocities, the check is for higher velocities when stripping is and issue and acts as the gear for the boolit that engages the "teeth" of the rifling.

However each rifle/load combination is a law unto itself and only the target will tell.

At one of my 25 yrd indoor clubs we shoot military rifle bench rest competitions with down loaded ammunition. There are a lot of .303" shooters and the standard recipe is 4-5gns of any fast pistol (source Harris) powder under a Lee Soupcan 115gn GC boolit, every one shoots this boolit unchecked to good effect but my No4T scatters them all over the paper without one but gas check the boolit and it settles down to nice gong collecting groups.

HTH.

BTW I'd try something a little softer than Lyman No2 as well.

If you do need to use gas checks they should ideally be crimped into place, you may be lucky and get a nice tight snap fit if not then a spot of cyanoacrylate glue will hold them in place. Your checks do need to stay on the boolit for any semblance of accuracy.

Thanks Adam. I'll give them a whack first and see what transpires.