How tight is too tight?

[MotelAlpha]

I have a new mold for my double rifle, and it casts nice 650 grain x .620 diameter bullets to match my .614 x .621 grooves and lands. Before I was loosing considerable accuracy with a mold producing .615 diameters. So far I have not shot the new slugs, and wonder since these go into the cartridge with a lot of pressure, have I gone too far with increasing the diameter? I definitely do not need to crimp these at all, where I had to put a real hurt on the old ones to keep recoil from moving them. I cast with WW and do not cold drop them, but let them cool at ambient.
Thanks all!

[JSnover]

A .620 boolit is actually undersized if your grooves measure .621 but I'd try them anyway. The rule of thumb is .001-.002 over the groove diameter.
If they're too hard to seat, try expanding your case necks a little more.

[MotelAlpha]

These are strait neck cases with no taper, or inside dimension changes, so a neck size expander is not used.

[texassako]

These are strait neck cases with no taper, or inside dimension changes, so a neck size expander is not used.

Do you just load the fired cases unsized? If so, you may have an issue with the chamber neck not excepting a bullet large enough for the bore if the .620" does not work. Otherwise, you expand after sizing and seat the size bullet you need.

[MotelAlpha]

Yep, I thought of that, and will have to assess what to do once I fire some of these. I am thinking the pressure will go up a bit, so will shoot some reduced loads first.

[retread]

You may want to pull one of the "too tight" ones and see how much you have reduced it's diameter. It will happen!

[williamwaco]

Measure the diameter of a bullet before you seat it.

Pull it without firing it and measure it again.

If your cases are that tight you are very likely sizing the bullets even smaller when you seat them.

That is not a good thing since your bullets are already undersize.

If you don't have an expanding die you should consider getting one.

[MotelAlpha]

I have decided to make a new mold, of .618, and see how well that works for me. It should be better than the small one......Thanks all

[Doc Highwall]

It is a lot easier to make a mould larger by .001"-.002" and size the bullet and lube it then make a new mould.

The first three rules of cast bullet shooting are, bullet fit, bullet fit, and bullet fit! The bullet needs to be large enough to seal off the powder gases behind the bullet and are typically .001"-.002" larger then the groove size.

My Remington 40X target rifle slugs .308" and I shoot .310"-.311" diameter bullets with great accuracy. My best five shot group at 300 yards is 1.610" with four of the shots in 1.090" with many groups under 3". My expander is .309" giving .001"-.002" neck tension on the bullet.

I and others hear have shot lots of straight wall cartridges like 38-55, 45-70, 45-90, and 45-110 and with sizing the cases use a expander like a Lyman M-Die or other brand for great accuracy and your gun needs to have the case necks expanded to give the right amount on neck tension also.

[blaser.306]

I cannot beleive that no one else has asked yet! Have you measured the ID of a piece of brass that was fired from your gun to see what diameter of boolit it will accept? And as far as being a "straight walled cartridge " even straight walled pistol when sized will be reduced below actual od when full length re-sized , then expanded up with a case neck die or "m" die to allow for proper neck tension. And alot of "straight" rifle cases actualy have a fair bit of taper to them from base end ( either rimmed or rimmless ) to the case mouth, so are not really straight at all , just no neck. what is the actual cal designation of your cartridge?

[MotelAlpha]

A good question. The inside diameter of these are .615. Tom at RMC makes his brass quite thick which is why I had him make me some ...This is for a .600, using reduced bullets (650 grains) and reduced powder loading.
recoil is still significant at 1800+fps.