CAUTION: The following post includes loading data beyond or not covered by currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.
4198 was one of the powders Eugene Stoner worked with in developing the AR platform. 50 grain bullets initially, I think. It is considered a bit fast today and not to produce maximum velocity, so the gas port might be a tad small for it with the light bullets (dwell time too short due to light bullet velocity) but it should still be possible to make it work for you. Several points to consider:
The 5.56 NATO chambers usually are a little more generous at the breech end and have slightly longer freebore than a .223 Chamber, so pressures in them are a little lower for equal loads.
Some 5.56 NATO cartridges (M855 and SS109) are loaded to about 6% higher pressure on our conformal measuring system than M193 was originally, so the gun and chamber work fine with that. It looks to me like you would get there at about 21 grains of powder if you were shooting Hodgdon's .223 test gun. But you aren't, and in addition to the difference between .223 and 5.56 chambers, a test gun has the very tightest SAAMI standard chamber, so it gets the highest pressures. The ballistics test technical handles the test rounds so the powder always falls over the primer for maximum pressure, where the self-loaders tend to throw it forward in the case which reduces velocity and pressure unless the case is well-filled by the powder charge.
You are using a different primer. One of the mildest ones made, where Hodgdon is using a Winchester primer which is likely a little warmer (I haven't shot the two side-by-side to see if the Winchester produced higher velocity from the same load). In 2006, Charles Petty showed that primer choice can affect velocity by up to the equivalent of about 5% difference in powder charge in the .223 shooting a 55 grain bullet. If, however, you decide to go with one of CCI's #41 primers made to mimic military sensitivity, then you would want to back down a grain as those are magnum primers and noticeably warmer than the ones you have. But switching primers is another thing you can try.
If you switch to H335, you may need the magnum primer to get the powder to ignite consistently for best accuracy. CCI magnum primers were reformulated to light those older formulation, high deterrent coating ball powders in 1989. Hodgdon confirmed to me the formulation of H335 (canister grade WC844) is identical to what it was when it was first produced in the 1960's.
The bottom line is I expect you would have no issues working up to 21.5 or maybe even 22 grains with your current primer, though I would not expect to change primers without working the load up from lower levels again. Since I'm not there and can't inspect your cases for
pressure signs, I have no way to be sure. It's just that for now, we know the loads feel mild and that the gun isn't cycling, and those facts say that between the chamber and primer differences you most probably have rather lower pressure than the published data have lead you to expect. Also, Western now publishes loads aimed at 430 MPa (62.366 psi) pressures claimed for European NATO rounds. This is probably at least part European vs. American measuring system artifacts and I got into a long discussion with them about it at the NRA Annual meeting, but was happy to conclude they were being more careful than I originally gave them credit for. In any event, they are apparently getting no damage reports, and, abstractly, that makes sense as the gun metal tolerates .308 and other rounds that work in that pressure range just fine. It would just be excessive gas port pressure and added throat wear that would concern you, and if no damage complaints are coming in, then these probably aren't a risky issue at that peak pressure.
Incidentally, military ammunition is required to be within not only pressure limits in the chamber, but within other pressure limits at the barrel gas port and to be within velocity limits that are about three times tighter than SAAMI limits for the same bullet weight. That's how they guarantee function.
