The clean out screw on my GPR takes an Allen wrench. Along with ant seize, never had a problem getting it out.
Clean out screw......That is what its there for, right? So I use it.
What that screw at the end of the drum or on the side of the patent breech is a manufacturing shortcut to drill the flash hole from the nipple seat to the powder chamber. And after the flash channel is drilled a screw is installed as a plug. It is handy to give it a new function and recommend the removal of the plug to facilitate cleaning. Note: After T/C replaced so many of the "clean up screws" on their breech, the process to drill the flash hole was changed and there was no longer a clean out screw. Some original drum and nipple percussion breeches have the "clean out" screws. Many do not. If proper care is taken, removal of the plug and nipple will make access to the flash channel for cleaning more acceptable. Be sure to always use a never seize lubricant on the threads of the what is now a clean out screw and the nipple. It is important to properly tighten these threaded items to the snug level of torque for easy removal when removal is warranted.
So Ames is correct. Once the rifle is completed and determined suitable for use, that screw is there as a clean out screw if you wish to use it for that purpose.