300BlkTalk

Generally speaking, How much of a seating depth change would equal roughly .1 gr of powder?

For instance, if you were to seat a bullet deeper by .030", would that equal an increase in powder charge by .1 gr?

Demphna2 wrote: ↑Mon Mar 20, 2023 12:37 pm Generally speaking, How much of a seating depth change would equal roughly .1 gr of powder?

For instance, if you were to seat a bullet deeper by .030", would that equal an increase in powder charge by .1 gr?

Too many variables. Powder density, load density, burn rate will really change the answer.

296/H110 when you get near the max load, 95-100% density, 010” depth and .1 grain is pretty close. But at 70% fill it’s completely different. For that reason subs and supers will have completely different answers.

Is the bullet seated below the neck or not?
The ratio change if then powder fill is in the neck or body, different diameters.

You would have to pick one specific load and work it out.

Found the answer through testing.

The basis for the question is when using a magnetospeed, sometimes it acts as a tuner. Meaning, you can have a great load with really good numbers and it shoot great with the bayonet on, but loosens up a bit with it off. I've seen the opposite effect as well, where a load will shoot loose with the bayonet on with really great numbers, but tighten up with it off.

From the little experimentation I've done across a few different rifles that acted this way, generally speaking, seating a bullet .020" deeper usually tightens things up.

In this particular scenario, using a fast powder for a subsonic load, some variables were at play that normally don't apply to a regular load workup. In order to get full function, seating the bullet further out helps with BHO. In my case, I was maxed out on how far I could seat the bullet out and getting full function. Seating deeper by .020" was not an option.

Testing .1 gr above and below the original charge weight showed some interesting results. .1 gr below grouped similar to the original load and groups were opened up (which I expected). .1 gr above tightened things up significantly indicating, to me at least in this scenario, that a .1 gr increase in charge weight is similar to seating .020" deeper.

I also played around with Gordon's reloading tool and that seemed to show the same results with this particular bullet weight powder combo.

Group with bayonet


Group without bayonet on


Increase in charge weight by .1 gr