1850s-60s:
I'd go for a S&W No1, a couple Colt Walkers, the first Henry rifle, a spenser, and a Sharps. I'd also try to get a couple of the Confederate copies of Colt's revolvers. I believe that very few of them were made, hence rare and very valuable. Davy Crocket's muzzle loader if possible.
In the 1910s-30s era, a Thompson 1927 (I forget the exact model), Colt 1911, S&W registered Magnum, and a first run Browning BAR. A couple of Holland and Holland double rifles, fancy engraved shotguns etc.
1940s-50s a Grease gun, Sten, Sturmgewher (sp?) and a MG42, S&W 44 Magnum, anything belt fed.
I'd also try to get stock in S&W, Colt, Winchester, Remington, Browning, Ruger, etc. Just give me $500 worth of gold to bring back to the 1800s or early 1900s.
I doubt that I'd change the political situation at the time regardless of how appalling it is to us in modern times. The consequences of altering history might seem an improvement however, if such evil actions as slavery, assinations of a President etc, you don't know if a greater evil would have filled its place. Right or wrong, I believe that both the accomplishments and the sins of this country have made this country a great place and a beacon of hope and freedom to millions of people.
Trying to influence the Amendments to the US Constitution would be rather delicate in my opinion. The Federalist and Anti-Federalist papers were written to convince the common person to adopt the Constitution or not. Stronger language for any part of the Constitution could result in none of it being adopted.
-Jim