I think I've decided to go PTR 91. I've found a 16" PTR 91 GI at an online dealer at a good price. In the description of this rifle it lists the trigger as "navy type polymer group". On the PTR site they have both "navy type polymer group" and "H&K metal trigger group housing". What is the difference in these 2 trigger housings and which one is preferable? If the metal trigger group housing is more desired could I swap the navy type polymer group with the HK metal trigger group housing easily? Also curious if swapping the fixed stock with an HK collapsible stock is relatively easy? I look forward to all your replies. Thanks guys.