Which M17? The Enfield Match rifle, or the Bushmaster AR style? The Enfield was used to create a VERY neat supressed carbine known as the DeLisle- in 45 ACP. The pressures of a .243 are MUCH higher than a .45 ACP, but bolt face does need modifying (and extractor) to handle the auto pistol case. Yours may have a modified extractor now- but check head diameter on .45 ACP and .243- I'm on the road, and don't have calipers w/ me.
What we have heah is.... failure to communicate.