BHN and Obturation

[GregLaROCHE]

I am shooting a Marlin .45-70 from the mid eighties. It has a micro groove barrel. That means the bore diameter takes up most of the space in the barrel because the grooves are very thin. In what is considered a normal barrel, the grooves are much larger.

Therefore, a lead boolit has more space to obturate. Some of the lead can move into the grooves. Does lead move in other directions also? The boolit probably elongates too, if there is not enough space for the excess lead to go into the grooves.

If a higher BHN alloy is being used, it seems like it would take a higher chamber pressure for the boolit to obturate enough to pass through the barrel. Is one to three thousands over bore still advised with a hard alloy?

I am particularly concerned, because my micro groove barrel has even less space that what is considered a normal barrel, so it can only change in length.

What do others think about this?

[Larry Gibson]

You are over thinking the "problem".

It is generally recommended to use a "harder" alloy with micro-groove barrels but that depends a lot on the velocity and initial size of the bullet to groove diameter.

Bullets that are over groove diameter are swaged down to fit the groove/bore size. "Obturation" generally means to expand ("bump-up" as it is commonly called) to fill and seal the bore. Also if your bullets are lubed the bullet will ride on a thin layer of lube. That means the bullet is swaged down to less than groove and bore diameter. That also means the bullet will indeed elongate to a degree.

As to the harder cast bullet alloy increasing pressure with a given load any increase occurs within the 1st bearing length of travel. That is early in the pressure rise and most often any difference is well within the ES of the psi anyway.

Cast your bullets, work up a load within published start/max loads, test the load work up and when accuracy isn't there or goes south the micro-groove barrel is telling you what it likes or doesn't like.

[HangFireW8]

Hard versus soft lead alloy has very very minimal effect on (over)pressure with unjacketed lead alloy boolits. Virtually every other factor matters more. If you work up loads as normal practice, you'll be fine.

Yes, MG 45/70 accuracy is usually found around 460-461. But 45/70 is a cartridge where softer lead and faster powders can be used to expand and obturate, so those diameters are not a hard and fast rule.

You may find some standard molds and dies and expanders make it hard to get to, load and stay at anything above .459. You may end up lapping or ordering some more niche equipment.

[243winxb]

Soft lead bullets skid and strip in rifling. Correct diameter and harder bullets are a plus, as is a gas check to help grip the thin micro.

Soft, near pure lead will cause bullets to slump. The lube grooves get smaller , as the bullet length gets shorter. Like hitting a cast bullet with a hammer.