Modifying a GC mold to a plain base design

  • Last Post 16 August 2017
reggleston posted this 10 August 2017

Anyone out there ever modified a GC mold to a Plain base design? Considering milling the GC depth from a GC mold and making a Plain Base  design out of it. Mold I have in mind using is a RCBS 257-120 2 cavity mold.

R D Eggleston

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David Reiss CBA Membership Director posted this 10 August 2017

I have a half dozen or so molds that have either had both cavities or one of two modified. 

David Reiss - NRA Life Member & PSC Range Member Retired Police Firearms Instructor/Armorer
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R. Dupraz posted this 10 August 2017

I have done the very thing that you ask but for a different reason. The mold was a two cavity NOE 22 cal PB that cast 60 grn. bullets and wanted to shorten the blocks for a shorter casting. As I recall about .050" or there about. 

I have a bench mill/drill and was reluctant to try it with this machine but threw caution to the wind and did it anyway. The hardest part of the process was the time spent in set up with a dial indicator, a machinist level and square in order to get the blocks perfectly square with the quill of the machine for a square cut.

The end result was that everything worked out fine and now the mold  drops a casting at about 55- grns , is within the parameters of a 14" twist and shoots well in my 218B contender pistol. .   

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OU812 posted this 10 August 2017

Before modifying mold, I would cut the gas check shank off bullet. You can do this in a drill press using a small fine tooth hobby saw  used for cutting balsa wood.

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MarkinEllensburg posted this 11 August 2017

My father did this exact thing or paid someone to do it to a pair of double cavity molds. So far I've not had much success with the bullets in my #1 .2-06; so far. I also have on the back burner two other cartridges he was working with. .25 SS and his wildcat the .25-223 RKS. Given this the molds are not for sale but perhaps I could cast a handful for you.

What is giving you the idea to try this? I'm curious. It is possible that my dad was inspired by Bev Pinney. He more than once told me I should try shooting plain base or even GC bullets without the gas check, at moderate velocities of course.

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Ed Harris posted this 11 August 2017

I had Erik at bore out the GC heel of a .30 cal. rifle mold for me to make a 7-1/2 degree boattail, plainbase.  It shoots VERY well in the .30-'06 with 6 grains of Bullseye.



73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia

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Ken Campbell Iowa posted this 11 August 2017

also consider that possibly the bullet might shoot  fine just by leaving off the gas check ;  as good as milling off the mold shank .

this has been a recurring topic by casters for years ... most report that the gas check shank castings shoot as well as the full base design .   but given the problems of testing even the same casting session of bullets 2 days in a row, i am not sure that is a guaranteed result.  

one advantage to leaving on the shank ... of course that you could always put the checks back on ... ( g ) ...


if the bullet at a long 120 gr. is twist unstable an option would be to facecut it ( 1/4 inch ?? ) deeper and then bore the base for a longer solid  base .  btw i would probably set the blocks up in a lathe 4 jaw and facecut it... triple indicating it true, of course.

just some thoughts... ken

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R. Dupraz posted this 11 August 2017

"tw i would probably set the blocks up in a lathe 4 jaw and facecut it... triple indicating it true, of course"


I considered that, but decided to try the mill to see how it would work. Using the four jaw in a lathe would probably be a better/easier way to go. If I ever do it again, think that's what I'll do. 


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reggleston posted this 12 August 2017

Just trying out different PB bullets for my 25-20 WCF that are in the 110-120gr range. Will try the GC bullet w/o GC before I do the modification to the existing mold.

R D Eggleston

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gnoahhh posted this 14 August 2017

I did it to a 311291 to make an ersatz 311241. I took reasonable care to square things up in the mill vise and it turned out ok, at least as good a shooter as a previous 311241. Biggest issue was the top alignment pin was intruded upon slightly, but doesn't effect things, as least not yet. Another minor issue was I managed to get too smooth of a cut which didn't allow very good ventilation under the sprue plate. Loosening the sprue plate didn't help much with the lackluster bullet base fill out. I took one even-handed swipe across the top of the mold with a coarse file and that cured it.

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JeffinNZ posted this 14 August 2017

ED: What is the purpose of the 'boat tail'?  Is it for ballistic purposes or loading?

Cheers from New Zealand

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TRKakaCatWhisperer posted this 15 August 2017

I'm not Ed, but I see 3 advantages.

One, ballistics - smoother contour,

two, easier loading (although not completely eliminating need to keep bullet from being shaved), and

three, if the OD at the base is bore diameter or slightly less, there might be a reduction of the 'finning' of lead (which might-could make it more consistent).

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Ed Harris posted this 16 August 2017

TRK hit all of the technical bases, but the REAL reason I did it was because the old NEI mold was worn out. By the time Erik refaced, squared the blocks and fitted a new sprue plate to cure its ills, the GC shank was too short, so he made the bevel.  I wanted this bullet for low noise, subsonic use in .30 caliber rifles with 10-inch twist and the boattail made sense.  The 7-1/2 degrees is what Erik came up with to neatly clean up the old base and GC shank.

The bullet has been absolutely splendid in every way.

Apparently I'm not the only one with this idea, as I found this bullet on Accurate's website.  I cannot take credit for it, but I like it:


73 de KE4SKY In Home Mix We Trust From the Home of Ed's Red in "Almost Heaven" West Virginia

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