TwoShadows,.... When talk'n 'bout the Lyman GPR's patten-breech, I haft'a rely soley on memory of the "one" time I ever had to remove one.
A young feller bought a GPR "kit" and asked to use my shop and tools to put it together. He did an excellent job with the metal and wood. When it was done, we commenced to sight it in by open'n the window above my work-bench which gave him access to the 100 yd. range in my back-yard.
After several attemps to fire the GPR with "hang-fires" (percussion rifle), I tolt him ther was sumpthin wrong with the "ignition",... we changed nipples, and tryed agin,.. still had "hang-fires",... we changed to "fresh" Goex FFFG and still got "hang-fires".
After try'n to blow down the bore without any success, I removed tha barrel and set it up in the big bench-vice to remove the breech-plug.
The "PROBLEM" was, thet the "flash-hole" in the bolster entered the patten-breech with jest barely a "crack" instead of a "hole". After remove'n the "clean-out" screw, I opened thet "crack" up to a "full hole" into the patten-breech. This was 'bout 5-6 years ago, and if I remember correctly, the patten-breech was 'bout 1 1/4"- 1 1/2" long and 'bout 3/8" in dia. (sure wisht I had a better memory, or had take'n some "measurements" when I had thet plug out),.... anyway, after I re-installed the breech-plug in tha barrel, the "problem" with ignition was cured.
Thet was the "ONLY" instance I've ever heard of, of thet "nature", and I've been around a "LOT" of GPRs. So to answer yore question 'bout use'n a breech-face scraper in Lyman's patten-breech,... YES, I think a feller could grind one down to "fit" if he was determined to do so. Personaly "I" don't think it's neccesary to use a scraper in a patten-breech, but would rather use the "flush-system" when clean'n. I cannot offer "any opinion", in regards to clean'n, when use'n anythin besides "real" BP in a patten-breech!!
Maybe someone else on this board knows the "exact dimentions" of the Lyman patten-breech, or has a better memory then I have, and can comment!!