I appreciate u/Georgy_K_Zhukov pointing me to this post; we've discussed this topic before, and I would probably have missed it without the ping. About a month ago on the 50th anniversary of the Gun Control Act of 1968, I wrote a long discussion about it on a gun subreddit, specifically for an audience interested in gun rights. This response is reworked from that set of posts to be more relevant to your specific question, suitable for an audience unfamiliar with the subject, and politically neutral.

The short answer is "yes," the NRA and other gun rights groups did oppose the Gun Control Act of 1968, which banned all interstate gun sales and non face-to-face dealer sales, effectively ending direct mail-order gun sales. The long answer is that the GCA was the result of a fight that had been playing out since the 1950s, with multiple interests on both sides, and involved a great deal more than just the mail-order prohibition.

If you want a very detailed discussion of the subject, David Hardy's article for the Cumberland Law Review on the Firearms Owners' Protection Act (passed in 1986 in response to what pro-gun groups and legislators believed was a pattern of excessive and abusive GCA enforcement by the ATF) is available online in its entirety. Hardy is an attorney specializing in firearms law who worked closely with the NRA and FOPA's Congressional sponsors to draft that bill and steer it through to passage, and his article goes into a great deal of detail about the Gun Control Act and its context. The article is, however, extremely dense in details and written in Lawyer (in some ways more of the story is told in the footnotes than in the body), so even in another sub I'd never just point you to it and say you're on your own.

Prior to 1968, state and local firearms laws were common and could vary very widely in their strictness, but the controlling federal gun laws were the National Firearms Act of 1934 (which severely restricted the possession of machine guns, silencers, and assorted non-pistol concealable firearms) and the Federal Firearms Act of 1938, which required gun dealers to obtain a federal license and to record all gun sales; and prohibited them from knowingly shipping firearms in interstate commerce to anyone indicted or convicted of a violent crime, or who lacked any purchase permit required by his or her home state. For individuals who had no criminal record and had the relevant state permits, it was easy to legally mail-order firearms.

The story of the GCA begins in the 1950s, when the postwar surplus boom cut into American gun manufacturers' sporting rifle profits. All the militaries of the developed world had spent years building and contracting for as many firearms as they possibly could, which included turning out vast numbers of full-powered bolt-action rifles. But by the end of the war, it was rapidly becoming clear that the future of military arms was the intermediate-powered self-loading carbine (this is the type of firearm we most commonly see today in the form of the AR-15 and AK families of rifles). Even in those countries slow to adopt the lower-powered cartridges, the automatic mechanism was still seen as essential. All those hundreds of thousands of bolt-action military rifles were suddenly completely obsolete and needed to be replaced, and many governments needed a way to liquidate them. With much of the west adopting stricter gun laws, the American market was a vast sponge of economic prosperity and relatively liberal gun laws, ready to absorb the seemingly endless stocks of yesterday's arsenal.

If you're Remington, for example, you're trying to sell a nice bolt-action Model 700 hunting rifle starting at around $95. At the same time, surplus importers like Hunter's Lodge are running ads in Popular Science that offer surplus military rifles for thirty-five bucks a pop, many of them already sporterized. A traditional deer rifle is fundamentally the same thing as a pre-WWII military rifle, sometimes even down to firing exactly the same cartridge; the military version will generally just have less careful fit and finish, a full length wooden stock under the barrel to protect it from damage, and generally no provision for optical sights (which were not used on mainstream infantry rifles, and would usually interfere with the fast-loading clip systems used in combat). "Sporterizing" a German K98, or American model 1903, or any number of other retired military rifles could be as simple as cutting the front end of the stock back to reduce weight, and possibly drilling and tapping the receiver for a scope mount. Even a nice sporterizing job on a surplus rifle would usually end up costing quite a bit less than buying a new purpose-built hunting rifle from a gun manufacturer, and those companies considered this import market an unfair form of competition.

Domestic gun manufacturers petitioned the State Department to restrict its issuance of import permits without success (indeed, in the immediate aftermath of WWII the State Department actively facilitated the mass commercial importation of surplus rifles and handguns, reasoning that those guns were safer in American civilian hands than arming international groups whose politics the US was opposed to). Finding no help in the Executive branch, they turned to the legislature. The first serious attempt involved attaching an amendment banning surplus imports to the 1958 Mutual Security Act. Opposed by the NRA, the amendment was defeated in the Senate.

But gun manufacturers didn't give up, and they split the pro-gun faction in Congress (some of whom were more loyal to the domestic industry than they were to the principle of free imports in particular). Through the 1960s they worked with Senator Thomas Dodd, a conservative Democrat from Connecticut (then home of over a half dozen major gun manufacturers) on a series of efforts to restrict surplus imports and mail-order gun sales, a cause in which they were joined by other representatives who favored gun control for noncommercial reasons. As they failed to pass in the face of resistance from the NRA and its Congressional allies, these efforts gradually increased in ambition, becoming full-blown replacements for the Federal Firearms Act of 1938. By 1968, the various proposals included lists of prohibited persons, import bans, assorted bans and restrictions on interstate sales, and the expansion of the National Firearms Act of 1934 to additionally restrict "destructive devices" (a category with several definitions and exceptions, but which can be understood as mainly weaponized explosives and non-sporting firearms with a bore diameter greater than .5 inches; along with common rifles and handguns, surplus dealers after WWII also commonly offered militarily obsolete surplus anti-tank rifles).