Th only fashion in which the NRA is a "lobby organization for the gun manufacturers" is because they're a lobby for gun owners...
Let's break this down further.
The NRA has lots of pieces and parts.
The part that I knew growing up was the guys who came and did gun safety classes for the kids in the summer. Most of us hunted and we were around guns a lot, so there were classes that our parents sent us to where we learned practical gun safety and how to take care of guns. The underlying message? Guns aren't toys and if you don't know how to use a gun properly, don't even touch it.
There were also a group of gun enthusiasts who held sporting events for marksmen- mostly small amateur events. This became bigger in the late 70s when the NRA started promoting
guns as a sport.
Membership dues supported the classes and the sporting events.
There's also a "legislative affairs" segment that has become increasingly active. There was a time when they advocated primarily for gun owners (and they still do). That has changed and they're advocating more and more for radical legislation that de-emphasizes the old approach of gun safety and instead pushes guns the way that Detroit used to push cars- not as tools for hunting/protection but instead as a sporty recreational item.
NRA claims to have about 5 million members- most of whom pay $30-40 per year.
In 2012 (the year before the Sandy Hook massacre of 20 children and 6 adults), the NRA took in $256 million. In 2013 (the year after Sandy Hook), they took in $348 million. If you do the math on the membership dues (5 million * 40), it doesn't quite add up.
That's because only about 50% of the NRA's income comes from membership dues.
The other 50% of the money is coming from "donations" to the political action committees associated with the NRA, advertising from gun manufacturers and direct donations from corporations. Individuals are limited to $5,000 per year in donations and the NRA says that most of the 30,000 individuals who donate are donating amounts less than $100- still not enough to account for the other 50%.
That's because the gun manufacturers- Beretta USA, Cabala's,Midway USA, Pierce Bullet Seal Target Systems, Springfield Armory Inc, Smith & Wesson, Sturm Rugar, et al are funneling money to the NRA via PAC donations, "advertisements" or direct donations (you'll see ads for manufacturers that say "10% of sales donated to the NRA"). Corporations don't give something unless they're getting something in return.
By the way, Wayne LaPierre's salary is over $1 million per year.
Oh -- about Republicans: the NRA's political donations used to be fairly even, but the Democrats have turned more and more against the right to keep and bear arms, so the donations have followed suit.
Let's tell the other part of the story...
They're giving money to Republican candidates because those are the legislators who are more willing to carry water for legislation that NRA and GOA pretty much writes. There's plenty of Democrats from Southern states who are supporters of gun rights and gun sports, but they're less likely to support some of the extreme legislation that the NRA has been promoting (e.g. armor-piercing bullets, stand-your-ground laws, removing restrictions on semi-automatic weapons, etc).
There are a few Democrats who are very strongly in true gun control- for example Rita Kestenbaum and Carolyn McCarthy (both of whom lost family members to gun violence). Most mainstream Democrats are in favor of more rational gun laws that favor responsible gun owners but limit access to guns whose purpose is to kill humans. The NRA spent $15 million running ads
against Democratic candidates.
Don't get me started on Gun Owners of America (GOA).