what gas check for .303 British?

I recently picked up a C312-185-1R mold.

Lee tells me

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You can use Lyman 226 or RCBS 465. We have no info on Hornady.

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but I don't see either as a part number on any of the online vendors I've looked at.

What gas check to I need for this?

.30 cal

regular 30 cal will do.
if they give you grief you may have to bump the size up a bit with a punch.

I have that mold, and I use .30 cal. gas checks. I also size without gas checks, and paper patch. I prefer that to using gas checks with that rifle.

Unless you have a minimum dimension bore you will probably be dissapointed whith that Mould. I had to lap and beagle mine to get the bullets to shoot at all and eventually i made a reamer and reamed it to a round nose design with a 304 diameter on the nose. From the box I never had any problems seating regular Hornady gas checks 30 caliber.

As has been said .30 cal GCs for that boolit,however you really do need to slug your throat/bore to know what the rifle needs.