So it doesn’t seem that the Army or the Marine Corps are in any hurry to explain to Congress why they don't use a common 5.56mm round.

The final joint version of the Fiscal 2017 National Defense Appropriations Act includes a provision requiring the secretary of defense to submit a report to the House and Senate Armed Services Committees explaining why the two services are using different types of 5.56 mm ammunition for their M16A4 rifles and M4 carbines.

The bill has already passed the House and is expected to be voted on and approved by the Senate this week before going to President Obama's desk for his signature.

This is not the first time Congress has gotten its dander up over this subject. Lawmakers asked both services to explain the same thing last year, but Marine Corps leaders said they need to do more testing of the Army’s M855A1 enhanced 5.56mm round.

I reached out to the Marine Corps yesterday and the Army today to ask about how they planned to deal with the request. I could almost hear the head-scratching as if neither service had heard anything about it.

According to the provision, the report must be submitted within 180 days after the bill, which includes the entire defense budget for the coming year, is enacted.

If the secretary of defense does not determine that an “emergency” requires the Army and Marine Corps to use the two different types of rifle ammo, they must begin using a common 5.56mm round within a year after the bill is passed, it states.

OK so here is the back story for those you out there who don’t know it.

The Army replaced the Cold-War era M855 5.56mm round in 2010 with its new M855A1 enhanced performance round, the end result of more than a decade of work to develop a lead-free round.

The M855A1 features a steel penetrator on top of a solid copper slug, making it is more dependable than the current M855, Army officials have maintained. It delivers consistent performance at all distances and performed better than the current-issue 7.62mm round against hardened steel targets in testing, Army officials maintain. It penetrates 3/8s-inch-thick steel at ranges approaching 400 meters, tripling the performance of the M855.

The Marine Corps had planned to field an earlier version of the Army's M855A1 until the program suffered a major setback in August 2009, when testing revealed that the bismuth-tin slug proved to be sensitive to heat which affected the trajectory or intended flight path.

The setback prompted Marine officials to stay with the current M855 round as well as start using the MK 318 Special Operations Science and Technology round developed by U.S. Special Operations Command instead. Commonly known as SOST ammo, the bullet isn’t environmentally friendly, but it offered the Corps a better bullet after the Army’s M855A1 round failed.

Since then the Marine Corps has purchased millions of MK 318 rounds.

The MK 318 bullet weighs 62 grains and has a lead core with a solid copper shank. It uses an open-tip match round design common with sniper ammunition. It stays on target through windshields and car doors better than conventional M855 ammo.

The Army quickly replaced the bismuth-tin slug in its new round with a copper one, solving the bullet’s problems in 2010, Army officials said.

The new Army round also weighs 62 grains and has a 19-grain steel penetrator tip, 9 grains heavier than the tip on old M855 ammo. Seated behind the penetrator is a solid copper slug. The M855A1 consistently penetrates battlefield barriers such as windshields more effectively than the M855, Army officials contend.

What is interesting is that the Corps was supposed to run tests on the current M855A1 round back in 2010. In 2015, Marine Brig. Gen. Joseph Shrader, then commanding general of Marine Corps Systems Command, told a congressional panel there were plans to test the M855A1 rounds again.

Military.com would really like to know what those tests show. We are going to continue to follow this story with great interest.