It does seem a bit motivated to choose federal lobbying as the initial representation of spending around the gun issue, when it's clearly dwarfed by initiative campaign expenditures, donations to individual candidates, and electoral campaign expenditures.
Do you have a source for this claim? They're listed as a nonpartisan organization online[1], and it seems as though their founders were a pair of Democratic and Republican senators. Their current lead project appears to be tracing the Biden administration's money, which doesn't feel like towing the line to me.
Edit: I found this conservative source[2], which refers to them as performing "left-wing" campaign finance advocacy. I think they're referring to the Citizens United decision, which has been a rallying point for every government watchdog that I know of. It occurs to me that (1) you could label just about anything as "left-wing" in this manner, and (2) campaign finance reform isn't something that most self-identified leftists would consider "left-wing" (being firmly in the "make government more transparent" category of political activity).
[1]: https://en.wikipedia.org/wiki/Center_for_Responsive_Politics
[2]: https://www.influencewatch.org/non-profit/center-for-respons...
When I go to that page, I get a few grafs on recent mass shootings (which, barring conspiracy theories, everybody is presumably on board with having happened). Mentioning those at all indicates that they have a viewpoint, but I don't see anything that indicates that it biases their transparency work. There's nothing to justify the insinuation.
For instance, they say that in 2020, there was $1,540,000 spent on gun control[1]. The Montana ballot initiative referenced above had $1,629,596.12 spent in opposition to in in 2020 alone[2].
[1] Committees in opposition to LR-130 https://ballotpedia.org/Montana_LR-130,_Limit_Local_Governme...
[2]TABLE: Gun rights vs. gun control lobbying, 1998-2020 https://www.opensecrets.org/news/issues/guns