I know this isn’t answering your question but wanted to share my experience. I bought voodoo tactical knees pads for my last archery antelope hunt. We averaged ~4-5 stalks a day crawling through brush and cactus. I wish I would have had elbow pads as well. I don’t think anything built in would have worked as well while crawling. For rifle you shouldn’t need all of that but something to consider as knee and elbow pads can be purchased cheap.
The cactus spikes in the sage is brutal! Even hunting rifle out there, I got down twice to shoot over my backpack and got punctured all over. I would think aftermarket Cordura knee pads would be in order for the crawl. Good fingerless Mechanix Wear gloves perhaps as well?