You did not mention the caliber that you are loading but seeing that it is H380 I feel certain that it is a rifle caliber. You also did not mention the load but if its much under the starting load given in the manual I would either go higher on the powder or switch to a different type powder. What you are seeing is delayed ignition and not detonation. Detonation destroys rifles (and sometimes the one shooting them). Believe me if it were detonation you would know it real quick like.
If you are trying to get a really reduced load or a squib load check some of the manuals and use some pistol powder at the charges recomended. H380 is not that slow a powder and usually detonation will occur with slow burning powders I would not take a chance. The labs heve never been able to duplicate detonation but in my old feeble mind there is enough evidence to support the belief that it does occur and I will not go much under a posted starting load. After 30 years in the oilfield and40 years reloading I still have all 10 fingers and I kind of plan on keeping them.
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Carlyle Hebert