The Marine Corps and the Army, for example, have a long history of buying into the same weapons, with some exceptions.
The Marine Corps decided in late 2017 to field concealable Glock 19M pistols, a variant known as the M007, to members of the Marine Corps Criminal Investigation Division in lieu of the M9 service pistol.
The Marine decision on the Glock came 10 months after the Army's $580 million contract award to Sig Sauer for the new Modular Handgun System as a replacement for the M9, made by Beretta USA -- a move that ignited speculation that the Corps would not buy the MHS.
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The Marine Corps' plans for the MHS, however, became clear recently in the service's proposed fiscal 2019 budget, which earmarks enough money to buy 35,000 Modular Handgun Systems.
"The Modular Handgun System will be purchased to replace the legacy M9, M9A1, M45A1, and M007 pistols with a more affordable and efficient pistol for maintenance," the document states.
It's likely that the Corps could not pass up the MHS price tag of $180 each, the unit cost listed in the budget document.
But this is not the first time the services have agreed to buy a common weapon. The Marine Corps, Air Force, Navy and Coast Guard decided to adopt the M9 pistol after the Army selected it to replace the M1911A1 .45 caliber service pistol in 1985.
Each service had participated in the meetings focused on the requirements needed in a pistol to replace the WWII-era 1911A1.
And with MHS, the services were involved in the "entire acquisition process, including source selection," Army officials said.
In addition to the Marine Corps, the Navy, Air Force and Coast Guard have placed orders for MHS, Sig Sauer officials said.
A similar move took place after the Army's decision to replace its M16A4s with the M4 carbine. The service first started to field the M4 to front-line units in the latter part of the 1990s.
The lightweight weapon's compact size makes it ideal working in tight quarters such as urban operations. The M16A4 is 39.5 inches long and weighs about 8.8 pounds, loaded. The M4 is 33 inches long with its stock fully extended, and 29.75 inches long with the stock retracted. It weighs about 7.5 pounds, loaded.
The Marine Corps began fielding M4s in limited numbers in the mid-2000s, but remained devoted to the full-size M16A4 rifle for several more years. The Corps finally decided to replace its A4s with M4s in 2015.
Leaders in the Pentagon don't seem to mind when individual services buy specific weapons in relatively small numbers, but the bean counters notice when a decision involves the purchase of hundreds of thousands of weapons.
The Army is the largest service, which is probably the main reason it is the Defense Department's executive agent for small arms.
There are exceptions to the rule, though.
For almost a decade, the Marine Corps and the Army have had their differences when it comes to small arms.
One of the more noticeable examples of this occurred when the Corps chose to replace the M249 squad automatic weapon in 2010 with the M27 infantry automatic rifle, a weapon based on the Heckler & Koch HK416 rifle.
Marines loved the lightweight, accurate M27. Army officials maintained that squads needed a belt-fed weapon like the M249 to quickly gain fire superiority over the enemy.
The Marines are so confident in the M27 that the service decided earlier this year to field the weapon to all Marine infantry units.
The Army instead has chosen to pursue a leap-ahead strategy that will focus on developing a next-generation squad weapon chambered in a new, intermediate caliber, such as the 6.5mm.
The Army argues that the current M855A1 Enhanced Performance Round will not penetrate modern enemy body armor plates similar to the U.S. military's Enhanced Small Arms Protective Insert, or ESAPI.
-- Matthew Cox can be reached at firstname.lastname@example.org.