The SR9 series were the results of HKs efforts to comply with the Bush "assault rifle" import ban of 1989. In the aftermath of the Patrick Purdy Stockton, CA schoolyard shooting, some politicians declared war on these "evil" guns and the relentless pushes for a ban began in earnest. In March, 1989, President Bush bowed to political pressure and declared by executive order that these guns were no longer legal to import. To accomplish this, ATF was left to decide what exactly was "non-sporting" about guns like the HK91, 93 and 94. They came up with features like pistol grips, flash hiders and bayonet lugs as being unsuitable for sporting purposes.
The SR9 series was the way of acceding to those new regulations. The HK91 was changed into the HK911 during the transitional period when importation of the HK91 was halted, but many HK91s were already in the country. HK Sterling stamped an extra '1' after 91 and this gun changed relatively little, other than it was now named something else, and not included on an import ban list.
The SR9 series consists of three rifles that HK imported for a time prior to 1994 and the Clinton Assault Weapon ban. They are:
The SR9: Basically an HK91 without the familiar flash hider, the pistol grip and traditional buttstock. Now present was the 'thumbhole' stock endemic to so many post ban semiautomatic firearms. Substituted for the traditionally rifled barrel was a new heavier polygonal bore barrel and enhanced buffer system.
The SR9(T): Simply an SR9 with a PSG1 trigger, pistol grip and an MSG90 buttstock. ATF deemed the target features to comply with the 'sporting purposes' edict, and allowed importation.
The SR9(TC): Simply an SR9 with PSG1 trigger group, pistol grip and buttstock. The forearm would not accept a bipod on any of these models. The standard HK91 furniture would readily install though. With a standard HK91 forearm and bipod installed, the gun became the rifle used by Billy Zane in the movie "Sniper" that I asked the trivia question about on the PSG1 page. All models are illustrated below.