I shot my new Sendero SFll 300RUM for the first time today, breaking in the barrel and trying out some Nosler 180gr E Tips. Have to go unleaded this year in my area in Ca. My starting load was 93 gr of retumbo Fed 215M primers and RP brass. Primers had small craters on everything from 93 to 95 grains. Nosler book lists Retumbo loads with 180 gr bullets from 95 to 99 grains so I didn't even come close to book max, keeping in mind Nosler told me to stay a couple grains under max. No other pressure signs, extractor marks, sticky bolt. I'm pretty new to hand loading and have not encountered this with my other two guns I've loaded for. I used a chronograph, velocity was around 3180 to 3265 fps with the different powder charges. My question is this, can something besides pressure cause primer cratering and how serious should this be taken? Thanks for your help.