Thread: Suppressors and pressure

I sorted out a preliminary load for my reasonably new 6.5PRC (sub 100 rounds), 53.5grn of H4831sc which is 1.2 grain under Hornady published max. This load showed the best accuracy with 143 grn ELD-x seated at mag length which was about 3/4" for three shots. ADG brass new, never fired. Plan was to use this load for the first 100 rounds (all of this projectile I've got) and then change to 147 ELDM (because I can get them) and develop a more refined load once the barrel isn't so new.

Initial load testing showed no pressure signs at, no extractor marks, no flattened primers, stiff bolt lift, nothing all the way to Hornday max (54.7 grns H4831sc).

Put a DPT suppressor on it (no change in barrel length and using factory thread) and when re-zeroing. Bolt lift was noticeably stiff and the brass (new ADG brass this time too) has visible imprinting of the extractor hole - not what I would think was dangerous levels of pressure but if I was testing for loads I'd probably stop there. Velocity was within 8 FPS of the average I noted during load development when no pressure was noted. 2820 FPS with supp vs 2828 FPS without previously - which is slow compared to published velocities. Accuracy was hard to say but two final shots were touching when confirming zero after adjustment so at least on par with previous testing.
I do note the first round was noticeably stiffer to open the bolt after firing so maybe it was because of a cold clean bore shot? But the extractor marks remained identical in subsequent shots (shot 5 to confirm/adjust zero).

I am surprised a suppressor changed chamber pressure to this degree as the max chamber pressures should be reached well before the projectile gets near the suppressor. Has anyone experienced a similar thing, the difference seems considerable where as I might have expected a very slight increase.
Or is there anything in case prep/rifle prep that I might have done? One thing I am thinking is possibly I didn't clean the resizing lube off the cases well enough compared to the original loads and I had higher bolt thrust rather than chamber pressure.