Young was out of bolts when I built my 243 LBC, so I'm using a good quality "standard" 7.62x39 bolt with a standard firing pin. I'm not sure why the pin would need to be longer? That's the first I've heard of that.
243 LBC is BHW's name for their version of the 6mm Grendel-based chambering, so you'll only find it from them. If you check their website (I believe the sales site is called "True Sporting Arms" now) they do sometimes have some 243 LBC barrels in stock; if you're lucky they might have what you want. I only waited 4 or 5 weeks for mine, but a buddy who ordered about the same time waited almost 3 months for a 6.5 Creedmoor barrel; that was early this year before they kept anything in stock. Both of our barrels shoot really well.
