I am finally getting around to loading for my .300.  I have had it together for over 6 months and decidedI should actually go shoot it.  Anyway, today I made 5 dummy rounds with Sierra 220gr Match Kings to make sure they would chamber correctly.  I loaded them into a 10 round mag and they were really loose, sliding back and forth.  I tried them in a different mag and they were tighter but not the tight I'm use to with .223 rounds.  What could be the cause and what can I do or is this somewhat normal?  I noticed in the really loose mag the bolt tends to just slide over the rounds instead of chambering them.  I've heard something before about bending the feed lips but I don't remember enough to just go tweaking things yet.  Thanks.

Nick

I use regular 20 and 30 rnd GI mags and the rounds fit without any slop in the mag, you shouldnt need to mod the magazine at all.  I am using Sierra 240g matchking .

What is your OAL on the loaded rounds?  

Quoted: What is your OAL on the loaded rounds?

I had to go down to 2.225 OAL to get them to chamber.  

I load my .300/221 to the same OAL as .223... about 2.20" - 2.25" with bullets long enough to be seated that far out.  Shorter bullets (125 gr. mostly) have a shorter OAL when loaded.  This has never posed a problem with either the longer or shorter OAL.

Your problem sounds mag related.  Get some new 20 rd mags (they work fine off a bench or bipod) or if you have a bunch of older 20 rd mags, go with Wolff mag springs.  

Scott

What's the magazine capacity of the .300 whisper compared to a .223? How 'bout bullet weight?

I use corbon 220 subsonic in standard GI mags with zero problems. I've read some guys lower the front lip of the mag a mm or two for the big bullets, but mines does not seem to need it.

Thanks, pretty cool. In regards to bullet weight, I meant like rounds per pound; i.e. 168gr 7.62 weighs ~ 20 rounds/lb. Just curious.

Sweet, thank you sir. Once I get out of school and start making bank I'll probably be mailing you a check for a upper.