Have both. For ease I'll refer to PAR and SAR to mean Pump Action Rifle and Side Action Rifle respectively. This discussion is limited to the small frame (223/300 BO) versions. Don't have a large frame one.
They both accept the following parts (as far as I know):
-Triggers The PAR has a quirky locking mechanism for the BCG that means only some triggers work. Check the orignal pimped out PAR thread for info
here. NB. Haven't had the SAR out to test but it appears that any trigger will work.
-Safeties
-Barrels Because there is no gas system you are going to want to cover up that gas port.
-Grips
-Optics
-Rail attachments
-Bolts Any standard small fram AR bolt seems to work, less the gas rings. It appears that different bolt faces like Valkyrie will work, so that's nice.
Neither seems to offer an easy rail swap
The PAR rail has cutouts for the pump mechanism. Per the thread linked above, you can cut the rail cleanly at one location and have a nice short pump action with the appropriate length barrel.
Buttstock options depend
For some odd reason Troy opted to use a fixed skeleton stock on the SAR and the PAR comes with either a side folder or a standard mil spec carbine stock. Regardless, all interface with the lower in a unique manner. I am not aware of anyone offering an adaptor besides Troy. So, you may be stuck with what it somew with until someone starts making new options.
Paging Rambones!!!!
The BCG appears very similar between models
But it is not standard. The cotter pin and cam pin appear normal. Replaced the latter with POF roller ones today and the action cycles fine. Have not fired. NB. The firing pin is unique, so no direct replacement is available.
Some additional observations
The PAR is easier to upgrade, and handles very nicely with a short barrel and collapsable buttstock. If you have the folding buttstock you will beed to lock it open if you want to shorten the barrel or you will be lower than 26" OAL. Hopefully IBI and MRA will offer appropriately sized AR barrels WITHOUT GAS PORT so people can easily swap these parts.
In both models the rail is retained by a milspec barrel nut, using a slotted rail that slides on and rotates into locking position. It is nowhere near as robust as say a Geissele rail. The PAR rail would be a huge PITA to replicate, so good luck. The SAR rail can be replaced, however the spring asssist which closes the action expands as the springs compress, so rail selection will be tricky. I got a Geissele mk14 mounted but the barrel nut was too long (versus the milspec) and even with an appropriate spacer the system would not operate. One could likely install the barrel and nut and then mill the nut to the right length and replace the OEM spacer setup with a new part, but it was not something I was interested in doing.
The original imports of the PAR had a railed forend that had some issues. A retrofit kit to the newer system of a pump similar to a shotgun was offered. Combining the two I have the increased reliability of the latter with the tacticool of the former. This does not appear to be an option now.
The PAR has no last round hold open, the SAR does. The PAR can be pumped back and left that way to show clear. The SAR does drop the BCG when pulled back and released as one would a side-charging AR.
Both are in no way a replacement for my AR, but both are quite nice for what they are, if expensive. As manually operated firearms, they are NR assuming you don't do something dumb and make a prohibited firearm.
