Manticore metric conversion

Use a 50mm pvc pipe for the barrel use a 25 mm (19.4mm inner diameter) pvc pipe for the gas tube, uses the same o-rings as the original model. everything else uses the original model files, the lengths of the gas tube is the same as the original model, no change was made on this part. FYI you can use M4 screws / bolts in place of the original 8x32 screws mentioned, but you will need to keep the nut from moving while tightening the screws as the nut holes are a bit larger than the metric M4 nut.