The probelms are caused by too soft case material or excessive chamber pressure or a combo of both for use is a fluted chamber firearm. Check with the ammo vendor on this issue or find a more suitable round that does not permit case flow into the flutes. The "flute marks" should be visible BUT NOT pronounced in that you can easily feel them standing out from the case walls. The/your MP5 will never work properly with this ammo and you will damage the extractor and possibly buffer with it. It has little to do with the heat in the chamber. It is the round.This is Winchester 124gr. +P+. Is it caused by too high a pressure forcing the brass into the flutes? We shot about 100 rounds of 124 gr standard pressure and no problems. We then fired 30 rounds of 115, light target loads and were having problems ejecting (not suprised) I ended with 30 rounds of this +P+, and the first 15 rounds went fine with no issues, then we started having failures to eject. Then failures to extract. Upon looking at the brass we found these which were the out of the last 10 rounds fired. Was the chamber getting too hot causing a rise in pressure?
Also. Once the extractor spring is "lame" caused by I susect in this case becuase the extractor was forced out of the bolt head bending the extractor spring when the bolt slamed forward on an empty case(s) during earlier stoppages. Once this occurs you must replace the spring or you will have failure to eject and extract with ANY round as the extractor is simply not getting enough spring tension to hold on to the case rim. This is the biggest design weakness in the MP5 and other roller-locked HK designs that use this style extractor spring. It was replaced in later guns (HK21E, PSG1/MSG90) but is not retrofitable in the earlier models. ALWAYS have a spare MP5 extractor spring close at hand because when they get bent they are done. If you can easily move the extractor with your finger replace the spring.