I had one, years ago--DST, with 3 barrels as I recall (223, 308, and a custom 358Win). I really wanted to like that rifle, but it never worked out for me.

The pistol grip and DST were long; many complained about this. I have arms and hands like an ape, so I rather liked it. The triggers needed work before they could be considered "good". The safety was like something designed by Ted Kennedy and approved by Diane Feinstein.

Either you disable the first stage of the safety, or you use two hands to take the safety off (forget about taking the safety off with gloves on). That was a total "FAIL" on a hunting rifle, at least for me--cost me more than one shot before I stopped using it. I swapped it off for something forgettable, and moved on.

Beyond scope mounts, I don't think there is much available for parts. On the other hand, I've not seen much call for parts--maybe they don't break, or maybe folks just tend to not shoot them. Other than the one I had, I've never seen one in the field or on the range. Custom barrels? The only way I've heard of is to find a factory barrel and re-use the monobloc.

If you get one, try to get the "magnum" action. They use a two-part locking lug, and will be marked "M" or "MAG" on the outside bottom of the action. (I've seen them both ways.)