This Bear Creek Arsenal AR-15 16" M4 contour 416R Stainless Steel with 6mm Arc barrel has a mid-length gas system with a 1:8 twist. Use a Bear Creek Arsenal ar barrel in your AR15 next build and experience superior performance back by a lifetime warranty! All finished barrels go through a 100% inspection process. Inspections Equipment includes CMMs and Air Gauges for the bores as well as visual and machine inspection. {{widget type="Magento\Cms\Block\Widget\Block" template="widget/static_block/default.phtml" block_id="42"}} Does not include gas block or gas tube