The MRP barrels are proprietary , which is the price you pay to have the slick monolithic upper and quick change feature
As already mentioned, LMT makes 5.56 barrels in 18" stainless, 16" stainless, 14.5" CL and 10.5" CL .
I looked into modifiying standard Ar barrel / making from blank but its not worth the hassle because the LMT barrels are very good and reasonably priced
Some complain that MRP barrels are expensive, but you need to compare apples to apples. Premium barrels ARE more expensive than the cheapies
If you want a new barrel check with Questars.
Unfortunately they can't import any more 14.5" and 10.5s for us regular folk ( civilians )