SR90,
First I need to explain that this is the method I use and it comes with a warning and I'm sure others are going to speak up in very strong words saying "if you do that you are going to blow your head off". I've been there done that on other lists and boards. Just so your aware, myself and many others use this method with great success from known gunsmiths, to serious and not so serious shooters. Yes it increases the bolt thrust somewhat but as long as your not pushing the pressures to begin with you'll be just fine. I've used this method on factory, Mauser, and custom BR actions alike.
Also I only use this particular method when blowing shoulders forward and the cartridge essentially doesn't headspace until it is fire formed.
When forming an Ackley shoulder, changing the shoulder angle, or blowing out body taper where the case headspaces somewhere before fire forming...... I use Red Dot powder, cornmeal settled in, and a wad of sh!tpaper to hold it in against the powder/primer good and tight. Chamber it a bang out the back window. You have a formed case without firing a bullet.
Blowing a shoulder forward method (for the Yogi case)
First off, I use almost full power loads. The reasoning here is, if it's not fully formed the first firing and you don't anneal afterward before firing again, it works the brass that much harder the second time. Can't put hard and fast numbers on this but I believe I have less case failures during fire forming going all the way the first time. In my original batch of 100 Lapua cases and the 60 or so Norma case I've formed in my 338 Yogi I didn't have a single failure on the first firing. I still have 93 or 94 of the original 100 Lapua case left with over 20 firings on them. More on this later.
Back to loads for forming.... Normally I just back off a grain or 2 and use them for my first couple of spotter rounds during competition so the round isn't actually wasted. Or the first round out of a cold/clean barrel which is a throw-away shot anyway. So when my brass gets quite few firings on them I'll order a new box of brass and prep it and slowly get it all fire formed 1 or 2 at a time, so when the old brass finally gives out (haven't reached that point yet with the Lapua stuff) your ready to go with the new once fired fully formed brass.
The second and most controversial thing I do is put a very light coating (can't stress how light is light here) of imperial sizing die wax on the body and neck of the cases just before firing. Just enough lube to put a shine on your finger tips. Then just spin the case body and neck inside of your fingers. The bullets are also seated out long with tight neck tension, to help keep the case pushed back initially, but once the primer is dented that method of holding the case back is no longer valid. I do it to make sure the primer is hit hard enough to fire because at this point your not headspaces at all on the case. The lube on the case stops the case from griping the chamber walls that very short period of time and stretching the case head back against the bolt face while the case body is still expanded out and gripping the chamber. With the light lube on it the whole case body gets setback against the bolt face and reduces stretching which in turn reduces stress, which gives you longer case life.
As a note I was able to somewhat prove this method without knowing it at the time. When forming some rounds once while testing at the NC 1000yd range, I forgot to pre-lube my cases. Well they fired and formed fine and didn't see any difference at the time. I was also unaware that I forgot to pre-lube at this point. I took them back home for match prepping the final time and put into my match case rotation. When I trimmed them I noticed the cut was really heavy on this particular batch of 10 cases. I measured the remaining cases and they were approx .010" -.012" longer than any of my other cases I've ever fire formed using this method. That's when it hit me that I forgot to pre-lubed them, because my can of die wax wasn't in my shooting box but on my loading bench. But at this point it still wasn't a problem until I started firing those cases. I had a split case on the 5th firing of one of those cases a month or so later. I figured... hey one case split out of all of these is no big deal. In the next 2 or 3 matches I ended up with 2 more split cases around the shoulder area. Now my curiosity was peaked. I number all of my cases so I know when they were fired and how many times exactly. All of those cases were out of that batch of 10 un-lubed cases. I have only lost one other case to neck splitting from the other 90 cases that were formed using the lubed method. But now 4 of the 10 un-lubed cases are retired due to splitting/cracking. I only use that batch for first sighters now in case one splits on firing and that shot drops out of the group, I don't want that in the middle of a record string in competition.
So the longer cases of .010" to .012" tells me that brass stretched that much without the lube in one firing. It had to come from somewhere. So to sum this up, other than the lubing and seating the bullet out, it 's a normal round minus 1 grain for me and you have a fully formed case.
Hope this helps, and you do as you see fit for safety reasons,
Good luck with your new toy. They are fun! Nothing like firing this 338 and watching the impact at 1000yds into the dirt bank. The difference between that 300 gr bullet as compared to even the 30s, 6.5s and 6mm bullets..... no comparison.
Steve
By the way magwell length is 3.75"
