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Burrowed from The Gun Zone -- SAAMI on 5.56 v. .223 Remington
The .223 Remington is rated for a maximum of 50,000 CUP while the 5.56mm is rated for 60,000 CUP. That extra 10,000 CUP is likely sufficient to cause a failure in a chamber that's only rated for the "sporting" .223 Remington.
The .223 Remington and the 5.56mm NATO, when checked with a chamber ream from a reliable manufacturer of each, also have discernable differences in the areas of freebore diameter, freebore length (leade) and angle of the throat.
45/70 work up with; 405 gr cast with lube bands, CCI 200 primers, Win
brass, old Unique [dirty] powder, chamber well lubricated between
firings with CLP for max bolt thrust.
Quickload calculation of pressure and velocity with 405 gr, Unique
shot 0) 15 gr, 24,700 psi, 1271 fps, no shot fired, listed for Quickload reference point only
shot 1) 18 gr, 33,217psi, 1403 fps, primer looks fine.
shot 2) 20 gr, 39,435 psi, 1485 fps, primer is fine
shot 3) 22 gr, 46,133 psi, 1563 fps, primer looks flat.
shot 4) 24 gr, 53,334 psi, 1637 fps, primer is flat.
shot 5) 26 gr, 61,067 psi, 1709 fps, primer is top hat and flowing,
action popped open and case ejected on firing
shot 6) 28 gr, 69,366 psi, 1778 fps, primer is top hat and flowing, case
head expansion .001", action popped open and case ejected on firing
shot 7) 30 gr, 78275 psi, 1846 fsp, case head expansion .0015" action
popped open and case ejected on firing, some leading in the muzzle.
shot 8) 32 gr, 87,841 psi, 1911 fps, case head expansion .0025", case head separating where case head is flowing into the extractor, action popped open, some leading in the muzzle, and case ejected on firing, primer is spreading out .010"
There was no change in headspace, but I had to stop the work up because of case head separation where the case head flowed into the extractor.
Reference loads:
1) "Loads for the 45/70" from the H.P.White laboratory via "American
Rifleman" 1950~1968 via "NRA Handloader's Guide" 1969 says:
405 gr Rem S.P., 17.5 gr Unique, 1286 fps, 25,240 pounds pressure, for 1886 Win
2) "Lyman's 47th" 1992 says:
400 gr cast, 16.5 gr Unique, 1286 fps, 27,000 cup, for 1886 Win
3) "Lyman's 47th" 1992 says:
385 cast gr cast, 17.5 gr Unique, 1411 fps, 38,500 cup, for Ruger #1
I know as a rule, you should never run 5.56 in a rifle chambered in .223. "BUT" the Handi-Rifle is a pretty tough gun.