Here is my scenario: I will be receiving a 26" barreled 338 Lapua in short order that will be throated at around 3.6 inches, I already have a slug of Lapua brass and H1000 powder to start my load development. I was planning on using the 300 grain matchkings and I expect the 300 grainer will use a lot of case capacity at 3.6"
I think the H1000 will fit the bill nicely with this setup, if I had a longer throat I think I would try using RETUMBO.
I want to get around 2750 fps with this rifle and the 300's.
Whatever you do just start low, work up and look for psi. If you seat slowly it will minimize case swelling and eliminate problems being pushed back out too.
I had problems using 250gr X bullets in my 338WM for that same reason. The bullets are long and when seating them on top of 72gr of IMR 4350 they would bulge the case if not seated VERY slowly and in little steps downward.
Powder was crunching from almost the beginning of the stroke. If I seated them in one stroke they wouldn't even come close to chambering because of the swelling just below the shoulder.
The powder would also push the bullet back out immediatly around .050" if not done very slowly too.
The Ruger would shoot them into less than a 1/2" hole but I knew pressure was on the high side with them going 3050 FPS on average using Federals HE brass.
Primer pockets lossened up after about 4 reloads, and when the brass was gone I already had my Ruger 416wby and stopped playing with it and sold it.
IMR4831 would get the same velocity but with only 70gr, I never played around with it beyond about twenty rounds though.
Bolt lift was always smooth but primers were flattened pretty good. No ejector groove marks were ever visible but primer pockets verified only after 3 reloads the psi was high, 10% of cases were pitched and wouldn't hold primers snugly to load a fourth time and after the fourth over 50% were unusable.
Normal Remington or Federal brass only yeilded about 2800 FPS with the same loads, so all I ever used was the 60 factory fired High Energy brass I had.
For some reason accuracy was just astonishing with these HIGHLY compressed loads.
If the powder is filling the case to or close to the mouth or above (meaning if you remove your powder funnel you will spill powder) try this.
Dump the powder into the case, using a funnel if required. Now "settle" the powder. For this I use an old vibrating case tumbler that uses a plastic bucket and lid (much like an icecream bucket). Simply dump the powder, and touch the base of the case to something soft (I use the plastic lid) of the vibrating mechanism. Poof! The powder just disappears into the case like magic!
If you don't have a vibrating tumbler, raid the top drawer of your wife's dresser or head down to your favorite sex toy shop and buy--you guessed it--a vibrator. Be sure it's one of those with a soft rubbery coating.
Ever noticed with (not severely) compressed loads, that some times they are compressed when you load them but after riding around behind the seat of your truck for a week or two you can shake a round and hear it has space? All that's happening is the powder is settling together instead of being "all stacked up" as it is when you first put it into the case. Gently vibrating the case a bit simply settles the powder to the bottom of the case before you seat the bullet.
I've been doing this for years and have been able to stuff some fearsome amounts of slow burning powders into various cases.
The ONLY problem I have ever had was when I was using way too slow a powder for a particular application and the loads were WAY too compressed. This was in a 300 Win Mag shooting 180's on top of H870. To slow a powder for the job. But I was able to stuff enough powder into the case to get 3200+ fps. They worked great for awhile--until the powder pushed the bullets back out of the case .050 or so.
But that was an extreme example. Using proper powders for the particular application, this shouldn't happen even if you compress the loads quite a bit.
You should adjust your vibrating machine (whatever that may be) and use a soft enough surface on it that powder doesn't bounce around at all. If it is, you have too much vibration and it's counterproductive. It should simply "flow" right down the spout smoothly.
Many do other things such as using "drop tubes" or lightly tapping the base of the case on the bench but I've found this to work much better.