Back when the Enduron powders first appeared a few years ago, Hodgdon assigned me to write an article on IMR4166 for their 2015 Annual Manual. I checked out their data and handloaded for nine rifles chambered in the .204 Ruger, .223 Rem., 5.56 NATO, .243 Winchester, .257 Roberts, .308 Winchester,.303 British, 8x57 Mauser, and .45-70.
Their data for the .303, 8x57 and .45-70 was all much lower-pressure than for the more modern rounds, and in fact the .45-70 data was for trapdoor Springfields. around 20,000 PSI--and in fact was shot in an H&H reproduction Officer's Model trapdoor. All the loads were consistent and accurate, as well as clean-burning--though there might have been a few unburned kernels in the .45-70 loads, just as there are with IMR4895 at trapdoor pressures. All the Enduron powders contain a decoppering agent, and it worked fine too.
They're also temp-resistant, and I tested that as well in both heat and cold. 4166 passed that test too.
My overall conclusion was that IMR4166 is an excellent powder, and just about as flexible as either IMR4895 or H4895, with a very similar burn-rate.
