I buy most of my .223 at wal-mart. Some times when I buy the box says 45 grain sometime 55 grain.

They are in the same box rem UMC at the same price. I have shot both through my AR and both work great. My question is what is the difference between the two bullets. I know 10 grain:).

Is it worth buying the 45 grain to put on the ammoshelf for storage?

Do you know the barrels Twist Rate ??? 1/12 1/9 1/7 ???

Usually the lighter grain bullets(45-55gr) preform better in a1/12 Barrel.

1/9 barrels do well with 55-63gr - But I alsodo well with 69gr with a 1/9barrel.

1/7 is usually best for 62-63gr and above.

Now I did use the word "Usually" a few times......That's because some guys claim they're using 45gr in 1/7 barrels and it shoots fine, You also say it seems to work fine for you - If that's all you can currently get for range fodder then great.....But if I was going to "Stock Up" on something I'd stick with the appropriateBullet weightthe barrels Twist Rate was designed for.

I wouldn't worry about rushing to stock up., Looks to me ammo is starting to become more available and I'd get the right ammo for the rifle at a decent price.....Have you checked : www.wide​ners.com ??? They just got in some IMI ammo in both 55 and 62 that'll knock your socks off. (If they still have it)

One of the biggest Mis-matches I've seen was someone shooting 62-63gr from a 1/12 Colt barrel...

The bullets were hitting the paper sideways at 50yds(Neat little outline of the bullets profile).....So in thiscase itmade a huge difference if you mis-match the bullets and twist - I'd think the same to be true going the other direction ???

Anyway 3 of my AR's are 1/9 to give me a wider variety of ammo choices(55-63gr), But probably wouldn't want togo below 52gr....One of these is a tuned up target rifle using 69gr handloads(Just about the upper limit for the twist rate) - It prints sub Moa @ 100 on Sunday's after a night of drink'in.

The 4th is my retro A1 build with a 1/12.....It would probably like 45gr ammo.

My bushy is 1/9 twist and I get tighter groups with the 45gr UMC than I do the 55gr of the same. That being said, if it shoots good go with it; 45gr is the smallest I can shoot accurately.

It all depends on the rate of twist of the barrel.

My barrel is a 1:9 twist, heavy match barrel on a Mil-SpecColt. It shoots either the 55 gr. or the 62 gr. very well, but then, that is what it was designed to do. Another thing to consider is whether your weapon is mil-spec or not, and if it is a brand name gun or an aftermarket knockoff.