A few things that need to be checked first.
[QUOTE="628jessejames, ]I found that using about 15 grains will allow me to keep it the same COL as my s&b rounds
Loading a different bullet generally requires a different COL. Even using the length from Hodgden may get you in trouble, since they list a completely different bullet also. There is a way to use the same seated depth, but you still need to make sure it will not hit the lands at that length.
I’m using the FL RCBS die for these. Unfortunately my scale is a cheap harbor freight model that doesn’t measure down to tenths of a grain.
If your scale will not weigh down to a tenth of a grain, 0.1, throw it in the garbage before you get hurt. Scales are only accurate down to the second to last number. A scale that will not read 0.1 will be read 1+/-.5, plus or minus 1/2 grain. Your 15 grain load could be 14.6-15.4 grains.
Is my scale the problem or should I try to hold the bullet out a little farther to prevent the case swelling?
Doubtful, unless you are seating really deep/short. Knowing what your COL is a must.
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First and foremost, what exact bullet are you using, or length. Speer makes three different 165 grain bullets, 2 are flat base and what is your COL?
At 17.4 grains the powder will not be up to the bottom of the neck, so that is not likely the issue. unless you are seating very short.
My guess is that it is more of a bullet seating issue than compressing the powder so much it's expanding the case. Brass may be too thick if it's converted, you can measure that and it should be less than .335". You can also seat a few without powder and see if they fit the gauge. If you are using a seating die that crimps at the same time or crimping too much are also good possibilities for trouble.
Side note, never start at the top of the load data.
Below is a link to one of the Speer bullets, it has the length listed, Is that your bullet or the other one?
https://www.speer.com/bullets/rifle_bullets/hot-cor_rifle_bullet/19-2035.html
This is Speer data for their 165 grain bullets, loading to that length is probably not a bad idea.
https://reloading-data.speer-ammo.c...e/30_caliber_308_dia/300_AAC_Blackout_165.pdf
Again pretty sure that it's not powder compression, and there are ways to confirm that also, but let's start at the basics.