Quoted: just went and put another 800 rounds through my colt making it a total of 1600 and there are no signs of breakage or cracks. (knock on wood)
which system/magblocks are having these problems?
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The bolt catch breakage issue is primarily with the 9mm style Colt bolt catch. But, I believe there have been one (or a few) isolated instances of broken 5.56 bolt catches when used in a 9mm Colt style set-up (these were posted about here, but they may be archived already).
Round count isn't a great way to track the potential for bolt catch breakage, because just shooting the firearm isn't doing anything to the bolt catch. What you want to be more concerned with is how many times you empty a mag and the bolt catch posp up to keep the bolt open. In other words, if you fired 1500 rounds using full 30 round mags, that is about 50 times that the bolt catch will get hit by the bolt and remain open. If you fired that same 1500 rounds, but only loaded 10 rounds per mag, your bolt catch would (or should) hold the bolt open 150 times, making it 3 times more likely to experience a failure.
The bottom load blocks (and the new dedicated top load Hahn block) that have the built-in actuator which is used in conjunction with a standard 5.56 bolt catch, absorb some of the impact of the 9mm bolt/carrier after it overtravels and reduces the likelihood of the bolt catch breaking. The standard top load Hahn block and the garrison block need the 9mm style bolt catch if you want to have last round bolt hold open. The easiest way to reduce the likelihood of bolt catch breakage with this set up is to reduce bolt/carrier overtravel, so the bolt/carrier just barely travels past the bolt catch.
The real cause of the breakage though is when the 9mm bolt/carrier overtravels after the last round is fired and then starts to come forward, it builds up enough momentum to impact the bolt catch with enough force to cause it to break prematurely. As you can see in the pivture below, the bolt catch is impacted at the top by the bolt/carrier (big red bar). The bottom of the bolt catch (below the roll pin) is actually pushed towards the back of the receiver. If there is enough "slop", the bottom of the bolt catch can hit the back of the notch it sits in in the lower, which puts pressure on it and can cause it to break below the roll pin (usually at the yellow line), causing the bottom of the bolt catch (in the purple circle) and the spring and pin beneath it to go flying. If you reduce the overtravel, you can reduce the potential for this to occur (or continue to occur) which is what is likely causing bolt catches to eventually break. Lowers with different specs in the bolt catch area can be some of the reason why some people don't have any problems and others seem to be breaking bolt catches on a monthly basis.
http://img.villagephotos.com/p/2004-12/908913/9mmbolt.JPGHopefully someone finds the above picture helpful.