Glock is protesting the U.S. Army’s selection of the Sig P320 MHS variant as the winner of the Modular Handgun System (MHS) contract to supply the military with 9mm handguns and accessories with a contract worth up to $580 million.

Firearms manufacturer Glock has filed a protest with the government over the Army’s award of a new pistol contract to competitor Sig Sauer, according to documents filed with the Government Accountability Office on Friday.

The move puts the Army’s plans to begin testing its XM17 Modular Handgun System on hold until the complaint is resolved.

The Army announced in January that it would replace the M9 Beretta, soldiers’ sidearm for more than 30 years, with a modified Sig Sauer P320.

Sig Sauer released the P320 in 2014; it is a polymer striker-fired pistol that has interchangeable grip modules that can also be adjusted in frame size and caliber by the user. And while the weapon can be adapted to shoot 9mm, .357 SIG and .40 S&W ammunition, the Army is opting to stick with the 9mm.

The service first announced the competition for the new pistol back in 2011. Sig Sauer beat out Glock as well as Smith & Wesson for the up-to-$580 million contract, which includes weapons, accessories and ammunition to be delivered over a period of 10 years.

The specific reason(s) for Glock’s protest (beyond not winning) have not been publicly released. Protests are common in the high stakes world of military contracts. There is no reason at this time to suspect that the protest will result in a favorable decision for Glock.

What is interesting, however, is that the fact Glock is protesting the decision strongly suggests that the company developed a “modular mystery Glock” that has not yet been seen by the public. None of Glock’s existing handgun designs come close to being “modular” as defined by the MHS contract, as the best they’ve been able to offer is replaceable backstraps. Sig’s P320 MHS  has an internal chassis system that is the serial-numbered “gun” (below) to which different grip modules and slides can be fitted in numerous calibers and slide lengths.

The Sig-built M17 contract is for two frame sizes and barrel lengths, but the Army can easily acquire commercial-off-the-shelf (COTS) kits from Sig Sauer in three calibers (9mm, .357 SIG, .40 S&W) and four slide assembly lengths and four frame sizes in three different grip diameters for different requirements, and has existing support for barrel devices, lights/lasers, and optics.

The government has until early June to respond to Glock’s protest.

The Army is expecting to begin receiving Sig’s first shipment of M17’s in roughly a month.