A LRBHO device is very convenient. It lets you know that your firearm is out of ammunition instead of suffering from some sort of malfunction or failure. However, it isn’t necessary.
What is a Last Round Bolt Hold Open (LRBHO)?
The LRBHO mechanism works by letting you know that your gun is out of ammunition. On the last shot a little tab on the magazine follower pushes up on a lever in the receiver.
This lifts the bolt catch into place and stops the bolt from traveling forward after the last shot. The bolt is left in the “locked open” position.
With enough practice, most people can tell when the bolt’s firing cycle is interupted by the LRBHO catch just by how the gun feels while shooting.
Sure it’s convenient, but do we really need a LRBHO?
Let’s look at a famous 9mm SMG that was designed without a LRBHO; the HK MP5.
It’s been reported that HK decided not to include a LRBHO on their MP5 on purpose. Supposedly, they found that most shooters didn’t notice the LRBHO activation in the heat of a conflict. They only reloaded when they eventually noticed that pulling the trigger did nothing.
Regardless of why the MP5 stopped firing, even if it was due to a failure of some sort, the “immediate action” process was the same (p. 29 of the operator’s manual):
- Gun fails to fire.
- Get behind cover.
- Lock the bolt back.
- Check for empty mag and make sure action is clear of jams.
- Strip the mag and replace if empty, or re-insert after clearing the jam.
- Close the bolt.
- Try firing again.
[Actually, they suggest transitioning immediately to a secondary weapon, e.g. sidearm, and only go through this “immediate action” procedure if no secondary weapon is available.]
Let’s compare this to a gun with LRBHO when the gun stops firing:
- Gun fails to fire.
- Get behind cover.
Lock the bolt back.- Check for empty mag and make sure action is clear of jams.
- Strip the mag and replace if empty, or re-insert after clearing the jam.
- Close the bolt.
- Try firing again.
There’s only one less step, but removing that step is a big time saver. With the bolt already locked back you don’t have to reach up with your non-dominant hand and open the action. Instead, a quick glance in the ejection port tells you everything you need to know about the firing condition of the gun (out of ammo or jammed).
Why would an AR-type gun not have LRBHO?
There are several reasons. Perhaps it uses magazines that don’t support LRBHO, such as modified UZI mags. Perhaps the manufacturer didn’t want to bother building it into the receiver or magazine adapter. Or, the action may be short-stroked to speed up cycling. When a 9mm AR is short stroked, the bolt doesn’t travel back far enough to activate the bolt catch.
What’s the process for an AR without LRBHO?
Without LRBHO, it’s a little more tricky with an AR receiver compared to an MP5. The bolt catch needs to be manually activated, but it’s on the left side of the receiver. It’s more difficult to quickly locate than the cocking lever on the MP5. However, the same basic steps apply.
A Magpul BAD lever or ambidextrous (ambi) bolt catch would be extremely helpful, since it allows bolt catch activation with the trigger finger.
For a 9mm AR without LRBHO, a procedure similar to the MP5 immediate action process would be:
- Gun fails to fire.
- Get behind cover.
- Pull the charging handle with the non-dominant hand while activating the bolt catch with the dominant hand.
- Check for empty mag and make sure action is clear of jams.
- Strip the mag and replace if empty, or re-insert after clearing the jam.
- Close the bolt.
- Try firing again.
This is essentially the same as the MP5 immediate action drill, except that instead of pulling the bolt back and locking the cocking lever in a notch, you need to push the bolt catch up while pulling the bolt back.
It’s also the same basic procedure that can to be followed if the LRBHO fails to catch the bolt, or some other “click, no bang” malfunction occurs in an AR platform gun.
It takes a little practice, but it can work just as well for the AR platform as the MP5 platform.
Another, slightly faster option, may be to do the following:
- Gun fails to fire.
- Get behind cover.
- Strip the magazine.
- Pull the charging handle back and check for jam. Move handle back and forth 2-3 times to clear the action if necessary.
- Insert fresh magazine.
- Chamber a round using the charging handle.
- Try firing again.
This procedure assumes that if there was a jam, the source of the jam was likely a bad magazine or bad rounds in the magazine. This avoids the manual use of the bolt latch mechanism. By removing the magazine first, the action is as open as possible for clearing jams, and if the magazine was empty it had to come out anyway.
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