Unexpected pressure signs

Recently, i improved the 6.5x65 RWS cartridge. Basically, blowed shoulders out to 6.5 creedmoor dimensions. For those of you that know, rws brass is VERY STRONG brass. I fireform using 48 gr 4831 with a bullet into the lands. Works pretty well. Anyway, when i started load development, i worked up 7828ssc powder first. Got up to 60 gr, cratered primers at 3340 fps. Loading 100 gr ttsx. First thought was powder TOO slow. Jumped to my short mag powder win 760. 55 gr, cratered primer, 3,450 fps. Now, thinking powder too HOT. RL-17 same tbing at 58 gr, 3,500 fps. Ended up settling on H-100v at 60 gr for 3,600 fps avg, but primers look like "moon craters"! I thought i might have a tight bore, but i sanded a bullet down .001, with same load, lost 100 fps. Im seeing all this that roughly 80-85% load density. I am now wondering if neck tension may be causing some problems. While the RWS only has .26 neck length, it is also .015-.016 neck wall thickness and pretty hard for virgin brass. My fired loads are expanding neck to .295 which is not too tight for chamber. Ive noticed that seating bullets is hard. It takes quite an umph to seat em. Wondering if neck sizing with .293 bushing vs .292 bushing will help. Any ideas out there?
Thanks

Thanks, all, for the replies. I miked the 6.5 creedmoor reamer that i used to deepen this chamber at .296 at the neck diameter, so i believe i have adequate relief for neck expansion during firing. When i reloaded my 270 wsm, the bullets seated much easier than this setup. I appreciate the letter, but my headspace is right at .001, and since i fireform weak loads, im not TOO worried about incineration. I will not crowd this powder another tenth of a grain though unless i figure it out. My fired brass looks fine, but if i crowd it much more, my firing pin will get a backblast from a cci! I am probably just asking for more than my design will allow. Im really fine with the velocity now. It equals 264 win with 8 gr less powder, but if i could relieve About 5,000 psi at that speed, i would love it! It shoots about .48 moa at 100 yds, which is good for my hunting. I will order .293 and .294 bushings and try it.

P.s. The edges of my primers are still rounded, but a crater. May have a weak firing pin return spring.

When i first bored this chamber out, i used P.O.'s method for chamber depth measurement, and set headspace at +.004 so it would chamber tight when fireforming. It was too much, as i had a "jelly filled donut". I pulled the barrel and took it down to .001, and that got rid of it, so no donut now after fireforming.

I took a bullet and pushed it into one of my fired cases, it didnt just fall down in the case, but i pushed it through with little effort.

Read an article recently about neck tension. In that article, they talked about neck wall thickness. They say that a case with .014-.015 neck thickness can actually have more "bullet grip" at .001 neck tension, than a case with .011-.012 neck walls, with .003 neck tension. I can drop one of these cases from eye level at an angle on a concrete floor, and it just barely dings the edge of the case mouth! I did that same thing with a win 270 case, and it folded the edge into the case mouth. Hmmm