Won't chamber....
 

Using SSK dies (Hornady make), .221 Fireball brass, 220gr SMK BTHP, 12.4g of H110, 1.360 case length, 2.216 OAL. The upper is a recent M1S upper, 300 Whisper. Factory SSK ammo chambers just fine.

Tell me, what am I doing wrong, how do I fix it?

Oops, wrong sub-forum. Would a moderator please move this to the proper sub-forum?

The M1S upper is NOT a 300 Whisper - it is a 300 Fireball (300-221).
It has very slight chamber differences so as to not step on SSK's Patent (?).
I also originally had that problem w/my M1S upper & had to set my 220 & 240g SMKs a little deeper to get the bolt to close.
How does your ammo compare to the factory SSK load as to COAL?

Oddly, my ammo is shorter OAL. I may just order a .221-300 die set from Redding - I've heard that works 100% with the M1S uppers.

That's what I'm using but I did originally have trouble w/220 & 240 being too long.
Everything else chambered fine.
I followed someone's suggestion to seat deeper & life is now good.

"1.360" Your case length is a little long. Trim to 1.340 I trim to 1.35 to be safe. Also check your headspace on the case. The dies need to be set to contact the shell holder to get the headspace correct. I have run into this before. Ron

If you trim your brass to the same length as SSK rounds and it still wont chamber first you will need to make sure the bullet isn't engraving the lands while trying to chamber. You would see engraved marks on the bullet one for each land when extracted if that was the case. In that case seat the bullet deeper. We have pretty good luck in making the parts we buy work out but sometimes the combination of parts result in tight headspace. I have a pistol barrel that, like Ron, I have to run the shell holder all the way up against the die to get it to size the case enough to chamber. Later I machined down a Redding shell plate so that I would have more control over how much sizing the die could do. If in the end you can't get it sized enough I can help you with a shell holder just let me know. I have Reddings competition shell holder set and thought it would have been nice if they had put a few -.00x shell holders in there along with the +.00x ones just for this reason.

An old trick I use is to cut the neck on a case & insert bullet. Load into chamber & ease the bolt closed(slaming the bolt closed may cause the bullet to hold on the lands). When you extract the round the bullet will be set at touching the lands & give you a precise OAL to the lands. I do this with all my rifles given a perticular bullet. Ron

The max. case length in my M1S upper is 1.400 - it's not the case, it's the bullet. Quite simply, the bullet is engaging the rifling before the bolt locks. I'm going to switch to 180 gr bullets for my upper and I should be A-OK.

Seat the bullet a little deeper!
I think mine was 0.20"????

The dies I'm using (SSK made by Hornady) can't seat the bullet any deeper.