Perhaps you missed an early post:
Likely they oppose unenforceable, ineffective laws.
In the 2010 report "Summary of Select Firearms Violence Prevention Strategies" the DOJ noted that “universal” background checks can’t be effective without a reduction in the illegal sources of guns to criminals and can’t be enforced without comprehensive firearm registration.
In "Source and Use of Firearms Involved in Crimes: Survey of Prison Inmates, 2016", the DOJ reported in Table 5 where criminals get their guns. We see that vast majority of guns in the hands of criminals come from straw purchases, family transfers, theft and the underground market (Illegal sources of firearms that include markets for stolen goods, middlemen for stolen goods, criminals or criminal enterprises, or individuals or groups involved in sales of illegal drug). A total of 0.8% come from gun shows. Purchases from "good guys" in private sales don't even show up.
What does a UBC do to prevent criminals from getting guns?
https://www.bjs.gov/content/pub/pdf/suficspi16.pdf