I have decided after much internal debate that I am going to stick with the load I worked up two years ago for my .300 Winchester Magnum utilizing IMR 4831 under the 180 Scirocco bullet.

When I worked up the load I did it with once fired/neck sized (lee collet) brass. I am using 74.5 grains of IMR 4831 in this load.

I have a couple of bags of new brass that is ready to load, and when I worked the load up originally I fireformed my new brass with a lighter charge and a cheaper bullet so I have never tried the load I worked up with "new" brass.

I have decided to load up 15 of the load in new brass and fire 5 three shot groups to see if there is even a point in worrying about fireformed brass for my hunting loads.

My question is this. I am sure I am getting close to the upper limits with 74.5 grains of IMR 4831. I have more room in my fireformed cases than I do in my new brass. Will the difference in capacity make enough of a difference in pressure that I need to worry about it?

NO. The difference is miniscule if measureable and of course the brass easily expands to fill the chamber. I'd say it's like worrying about a tenth of a grain difference in charge weight. I found my loads for hunting using new brass vs my reloads had no difference in POI or velocity.

300

If you change any component, switch lots of powder or brass you should reduce the load a few grains and work back up!! you are over max as far as reloading data.confused: that can be a formula for disaster

I'm shooting a 300WSM with a Hornady 180 grain Interlock and 64 grains of 4831 and getting 2950 fps. What kind of velocity are your 180 grn Sciroccos getting?

Averages around 3160 fps. I shoot a Browning Stainless Stalker with a 26" Tube.

Both Kevin and HIVELOCITY are right. I did the same experiment. I took brass from the same lot, 6 new, 6 once fired and loaded my favorite load which was at max for my rifle. No difference. However, brass can change quite a bit from lot to lot, and that will make a lot bigger difference than new vs. fireformed. I had an old lot of win brass for my 270 and compared the case capacity to a new lot, and the new lot had a greater case capacity. I wouldn't see pressure signs in the new lot like I would with the old lot for the same load. If the new brass of yours are from a different lot than the old, start 1 grain under your old sweat spot, maybe .5 under and then go up.

I haven't noticed a difference in new and neck-sized brass ballistically in my loads. I did read somewhere (can't recall the reference) that new brass would paradoxically LOWER the pressure as expanding the brass to fit the chamber was work taken from the expanding gas. Dunno if that's true. At any rate, if you're that close to a dangerous pressure, you're too close already. The difference is bound to be small.

While I agree the difference between new and fired brass is miniscule, there IS a difference in my experience. I've also been able to measure it on a chronograph.

Some years back my "good friend" Rick Jamison wrote in Shooting Times that pressure increased with each firing of brass, from new on up. He seemed to base it on the fact that brass work hardened. He did have some numbers in support of this. A fellow poster, a professional ballistician, had an Oehler M43 and after discussion on the subject, he ran tests and posted results. If memory serves, the test was run on a 30-06. He found that there was a difference of 3,000psi between new brass and formed brass, the formed brass having the higher pressure. In subsequent firings, after the first, there was no further increase in pressure as claimed by Jamison. From chronographing fireforming loads and loads in formed cases, notably in 338 Win Mag, I've noted as much as 40 fps difference between fireforming loads and subsequent reloads, particularly in domestic brass where belted magnum cases in the most part are notoriously undersized to the chamber. The higher pressure in formed brass would be expected to produce higher velocity.

Good reply BOB and Ricochet. I have read elsewhere of guys seeing the same results. Some say less powder is necessary in the fireformed cases, and some loads need to be tweeked slightly after fireforming since the energy is not needed in expanding the brass.